fix: Use shebang `#!/usr/bin/env bash`

[hyperupcall]

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This updates the shebang to #/usr/bin/env bash. It makes it compatible with not only FreeBSD and friends, but also NixOS.

QA'ing changes

make test

Fixes #385

[inkarkat]

@karbassi Welcome back; it's good to hear from you again.

I don't see any problems, but would suggest that we work on the backlog of PRs first. Especially my #405 has been waiting for your approval for a long time, and touches many files. If you attack the PRs latest first, that'll only cause merge conflicts.