nix-prefetch-hg: remove Bashisms

[toastal]

This could be POSIX shell which would reduce closure size & increase speed.

Related to #462164 where we actually could switch to dash over bash…

$ result/bin/nix-prefetch-hg https://hg.prosody.im/trunk 0ff11f2e87cd
hg revision is 0ff11f2e87cd
hash is 155cim431wv65a7inmdd0lhkr9gnzr5z0vl015ay5sqwqigcf07q
path is /nix/store/i0h4i7xjzh02qipqixl7j3ax91il77hb-hg-archive
155cim431wv65a7inmdd0lhkr9gnzr5z0vl015ay5sqwqigcf07q

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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Footnotes

1. Please consider giving up MS GitHub or offering a non-proprietary, non-US-corporate-controlled mirror for this free software project. I wish to delete this Microsoft account in the future, but I need more projects like this to support alternative methods to send patches & contribute. ↩