Auto merge of #123246 - Kobzol:tarball-reproducible, r=Mark-Simulacrum

Make source tarball generation more reproducible

This PR performs several changes to source tarball generation (`x dist rustc-src`) in order to make it more reproducible (in light of the recent "xz backdoor"...). I want to follow up on it with making a separate CI workflow for generating the tarball.

After this PR, running this locally produces identical checksums:
```bash
$ ./x dist rustc-src
$ sha256sum build/dist/rustc-1.79.0-src.tar.gz

$ ./x dist rustc-src
$ sha256sum build/dist/rustc-1.79.0-src.tar.gz
```

r? `@Mark-Simulacrum`

Author: bors