fix(ci): fix file paths in CI-generated *.sha256 files on *nix

[rami3l]

Fixes #3693.

See #3693 (comment) for more context.

The sha[256]sum command on *nix relies on CWD if you use a relative path as the input arg. All file paths in its output are then relative to it.

The PowerShell script doesn't need to be fixed since there's no path in the sha256 file at all.

Concerns

• How do I test this change? It does look quite correct though... I've tested locally. Although this is expected to print out

  f21c44b01678c645d8fbba1e55e4180a01ac5af2d38bcbd14aa665e0d96ed69a *./rustup-init

  which is not what @darthbanana13 originally asked for ¹, both shasum --check rustup-init.sha256 and sha256sum --check rustup-init.sha256 do pass. I hope this result remains satisfying.

Footnotes

1. According to https://superuser.com/a/1566139, f21c44b01678c645d8fbba1e55e4180a01ac5af2d38bcbd14aa665e0d96ed69a ./rustup-init is NOT a legal output. The file path after the separating space must start with either a indicating "text input mode", or an * indicating "binary input mode". Given this, it's quite reasonable to see an * in a sha256 file for a compiled executable. ↩