Motivation for this change

When cross-compiling, zstdless has /bin/sh as a shebang, while zstdgrep has build-time bash. zstdgrep is manually patched for running the tests, and the automatic patchShebangs after installation doesn't convert build-time bash to runtime bash.

Since we can't run tests while cross-compiling anyway, I moved the manual patchShebangs to checkPhase. This prevents it from interfering with the automatic patchShebangs fixup when cross-compiling. For native builds, build and runtime bash are the same, so this still works fine.

cc @orivej

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.