`pkg_rpm` cannot set different permissions to the executable in `pkg_files` and to its runfiles

### The problem

`pkg_tar` automatically detects the executable in `pkg_files`:
https://github.com/bazelbuild/rules_pkg/blob/c53ff51070c96b368188f057a6683d97c31b77f7/pkg/private/pkg_files.bzl#L395

And sets its execution bit:
https://github.com/bazelbuild/rules_pkg/blob/c53ff51070c96b368188f057a6683d97c31b77f7/pkg/private/pkg_files.bzl#L411

`pkg_rpm` does not seem to do anything similar, so executables end up with the same file mode as the rest of the files in the provider, which is usually a mode without the execution bit.

### Workaround

A workaround is to set the execution bit for the `pkg_files` target, which will set it for the executable and all its runfiles, but this is not ideal. It would be better if the default was to preserve the file mode of the source file, I think that would work reasonably well.

### Future work

This highlights a general issue with the `include_runfiles` attribute in `pkg_files`: because the executable and all its runfiles are grouped inside a single `PackageFilesInfo` provider, there is no granularity to have different attributes or prefixes for different files.

There is also the issue of executables being included twice, as the main target, and as one of the runfiles, which makes a warning show up (file mapped to two different destinations). This should probably have its own issue though.