The tarball staging path used `Copy-Item`, which on *nix doesn't preserve
the source file mode, so `pwsh` ended up 644 in the `.tar.gz`. The Debian,
RPM, and macOS PKG paths explicitly `chmod` everything to 644 and then bump
`pwsh` back to 755, which silently demoted `createdump` (the .NET helper
that produces crash minidumps) along with it. Now we `chmod 755` both
executables in all package staging paths, guarded by `Test-Path` since
fxdependent builds don't bundle `createdump`.
Also added regression tests which check the permissions of `pwsh` inside
the Linux and macOS tarballs before we upload them.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
