pkg_files remove the executable permission to generated executable

Say I have
```
cc_binary(
name = "foo",
....
)

pkg_files(
name = "foo_files",
srcs = [":foo"], # in practice it also include other files to become a group
)

pkg_tar(
name = "foo_tar",
srcs = [":foo_files"],
)
```

The foo file in the tarball will have permission 644:

https://github.com/bazelbuild/rules_pkg/blob/3917ae64448da126ded00197a4cc99ac43e2b20f/pkg/mappings.bzl#L258

It seems to me counter intuitive. Should the above line be None rather than 644? or am I misunderstood the usage of pkg_files?

Read pkg/mappings.bzl around line 258 and trace how pkg_files passes a cc_binary into pkg_tar. Reproduce the provided foo example and inspect the executable's mode in the resulting tarball. Done means the expected executable permission is preserved rather than being forced to 644.

Written by the indexing model from the issue text.