Update shebang to use env

[acmcarther]

This PR replaces "/bin/bash" with the more general "/usr/bin/env bash". This is necessary on platforms that don't have bash in /bin, namely BSD and esoteric linux distributions such as Guix and NixOS.

FWIW: This is a pervasive problem is Bazel. My own build rules for Bazel apply a multi thousand line patch just to let Bazel build on NixOS. I'm OK with this PR being closed if you feel this is too much of a niche problem.

[mfarrugi]

I would consider this strictly more portable, is this blocked for any other reasons?

[lilianmoraru]

The main maintainers are clearly inactive(a year or over an year).

@acmcarther If you are willing to become a maintainer, can you please ask to be added to the group( bazel-discuss@googlegroups.com )?
It would be really nice to see some of the pull request to get in...

[davidzchen]

Sorry for the wait. I have been the sole maintainer of this during my spare time, and due to commitments from my regular work and family commitments that have come up, I have not had much time to commit to this.

I have reached out to seek new maintainers, and @acmcarther, @mhlopko, and two others have graciously agreed to take up maintainership of rules_rust. I will help with getting the existing PR's committed and will then take on a more advisory role, focusing on helping the new maintainers get up to speed on the project and answering any questions they may have.

@acmcarther This change looks good to me. Can you rebase and then go ahead and merge? Thanks!

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

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Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this State. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[acmcarther]

Error was caused by a bad rebase that scooped in changes.

I undid the rebase, re authored the commit, and force pushed. Reviewing test failure now.

[acmcarther]

Test failure was elsewhere. Will merge with dzc's blessing in
#46 (comment)

[hlopko]

Sorry to be late in the party, I think this could be rewritten to use https://docs.bazel.build/versions/master/skylark/lib/actions.html#run_shell and get the same bash location as other rules in bazel are using.