[Bug] git-credential-netrc.perl is not built and is not available in `exec-path`

[Siavash <siavash.askari.nasr@xxxxxxxxx>]

which is located in `contrib/credential/netrc`.

If I'm not mistaken, it's because it sets the `SCRIPT_PERL` environment variable here:
https://github.com/git/git/blob/88dd4282d949cdafff516650c1be8aaf4d67983f/contrib/credential/netrc/Makefile#L13

But the Git Makefile un-sets environment variables here:
https://github.com/git/git/blob/88dd4282d949cdafff516650c1be8aaf4d67983f/Makefile#L587

Also credential helpers in `contrib/credential` are not present in the
output of `git help -a`, is that a bug?

I'm using Git v2.31.1.