refactor: reduce execution requirements for copy

It causes warnings like https://github.com/bazelbuild/bazel/blob/753cd459e49d1ffae1420bf5d46db228449299c6/src/main/java/com/google/devtools/build/lib/sandbox/SandboxModule.java#L514
and also makes toolchain selection broken for host!=exec

Fixes #466

.github/workflows/ci.bazelrc

@@ -9,7 +9,6 @@ build:local --disk_cache=~/.cache/bazel
 
 # Remote build execution
 build:rbe --extra_execution_platforms=@aspect_bazel_lib//platforms:x86_64_linux_remote
-build:rbe --genrule_strategy=remote
 build:rbe --host_platform=@aspect_bazel_lib//platforms:x86_64_linux_remote
 build:rbe --jobs=32
 

lib/private/copy_common.bzl

@@ -1,61 +1,11 @@
 "Helpers for copy rules"
 
-# Hints for Bazel spawn strategy
+# Hints for Bazel spawn strategy, copied from
+# https://github.com/bazelbuild/bazel-skylib/blob/0171c69e5cc691e2d0cd9f3f3e4c3bf112370ca2/rules/private/copy_common.bzl
+# See extensive comments there for reasoning on this execution-requirements selection.
 COPY_EXECUTION_REQUIREMENTS = {
-    # ----------------+-----------------------------------------------------------------------------
-    # no-remote       | Prevents the action or test from being executed remotely or cached remotely.
-    #                 | This is equivalent to using both `no-remote-cache` and `no-remote-exec`.
-    # ----------------+-----------------------------------------------------------------------------
-    # no-remote-cache | Results in the action or test never being cached remotely (but it may
-    #                 | be cached locally; it may also be executed remotely). Note: for the purposes
-    #                 | of this tag, the disk-cache is considered a local cache, whereas the http
-    #                 | and gRPC caches are considered remote. If a combined cache is specified
-    #                 | (i.e. a cache with local and remote components), it's treated as a remote
-    #                 | cache and disabled entirely unless --incompatible_remote_results_ignore_disk
-    #                 | is set in which case the local components will be used.
-    # ----------------+-----------------------------------------------------------------------------
-    # no-remote-exec  | Results in the action or test never being executed remotely (but it may be
-    #                 | cached remotely).
-    # ----------------+-----------------------------------------------------------------------------
-    # no-cache        | Results in the action or test never being cached (remotely or locally)
-    # ----------------+-----------------------------------------------------------------------------
-    # no-sandbox      | Results in the action or test never being sandboxed; it can still be cached
-    #                 | or run remotely - use no-cache or no-remote to prevent either or both of
-    #                 | those.
-    # ----------------+-----------------------------------------------------------------------------
-    # local           | Precludes the action or test from being remotely cached, remotely executed,
-    #                 | or run inside the sandbox. For genrules and tests, marking the rule with the
-    #                 | local = True attribute has the same effect.
-    # ----------------+-----------------------------------------------------------------------------
-    # See https://bazel.google.cn/reference/be/common-definitions?hl=en&authuser=0#common-attributes
-    #
-    # Copying file & directories is entirely IO-bound and there is no point doing this work
-    # remotely.
-    #
-    # Also, remote-execution does not allow source directory inputs, see
-    # https://github.com/bazelbuild/bazel/commit/c64421bc35214f0414e4f4226cc953e8c55fa0d2 So we must
-    # not attempt to execute remotely in that case.
-    #
-    # There is also no point pulling the output file or directory from the remote cache since the
-    # bytes to copy are already available locally. Conversely, no point in writing to the cache if
-    # no one has any reason to check it for this action.
-    #
-    # Read and writing to disk cache is disabled as well primarily to reduce disk usage on the local
-    # machine. A disk cache hit of a directory copy could be slghtly faster than a copy since the
-    # disk cache stores the directory artifact as a single entry, but the slight performance bump
-    # comes at the cost of heavy disk cache usage, which is an unmanaged directory that grow beyond
-    # the bounds of the physical disk.
-    # TODO: run benchmarks to measure the impact on copy_directory
-    #
-    # Sandboxing for this action is wasteful as well since there is a 1:1 mapping of input
-    # file/directory to output file/directory and no room for non-hermetic inputs to sneak in to the
-    # input.
     "no-remote": "1",
-    "no-remote-cache": "1",
-    "no-remote-exec": "1",
     "no-cache": "1",
-    "no-sandbox": "1",
-    "local": "1",
 }
 
 def progress_path(f):