do not preserve generated README's permissions

this removes the requirement of having the generated file already
present in the source tree. otherwise, if it does not exist, `cp` will
fail with "Permission denied".

the problem here is that `--no-preserve` is only guaranteed to be available
in `coreutils`, such that really this rule can only be run within
`nix-shell` or non-Nix Linux having `coreutils` installed.

core/docs/copy-files.sh

@@ -4,6 +4,6 @@ set -euo pipefail
 # copy files, passed as pairs of source and target locations, to the workspace
 # directory the calling rule is run from
 while (($#)); do
-  cp "$1" "$BUILD_WORKSPACE_DIRECTORY/$2"
+  cp --no-preserve=all "$1" "$BUILD_WORKSPACE_DIRECTORY/$2"
   shift; shift
 done