go_repository: fix missing default GOPROXY (#1602)

This helps unblock the Go 1.21.0 upgrade

internal/go_repository.bzl

@@ -115,6 +115,9 @@ Authorization: Bearer RANDOM-TOKEN
 # We can't disable timeouts on Bazel, but we can set them to large values.
 _GO_REPOSITORY_TIMEOUT = 86400
 
+# Default value from `go env GOPROXY`
+_GOPROXY_DEFAULT = "https://proxy.golang.org,direct"
+
 def _go_repository_impl(ctx):
     # TODO(#549): vcs repositories are not cached and still need to be fetched.
     # Download the repository or module.
@@ -237,6 +240,10 @@ def _go_repository_impl(ctx):
 
     env.update({k: ctx.os.environ[k] for k in env_keys if k in ctx.os.environ})
 
+    # Since Go 1.21.0, GOPROXY must be set to a non-empty value.
+    if "GOPROXY" not in env:
+        env["GOPROXY"] = _GOPROXY_DEFAULT
+
     if fetch_repo_args:
         # Disable sumdb in fetch_repo. In module mode, the sum is a mandatory
         # attribute of go_repository, so we don't need to look it up.