use bash when invoking dist shell scripts on solaris

[binarycrusader]

Partially fixes#25845

A separate, trivial fix is needed to the rust-installer scripts to completely resolve this issue.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[alexcrichton]

@bors: r+

[bors]

📌 Commit 087c233 has been approved by alexcrichton

[binarycrusader]

Please excuse the last commit I added; I reverted it. I'm still learning the process of GitHub's pull requests. I didn't expect those commits to show up on this pull request.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 087c233 has been approved by alexcrichton