treewide: fix shebangs - /bin/bash -> /usr/bin/env bash

[sorki]

Most scripts/tools use non-portable /bin/bash, switch to environment agnostic /usr/bin/env bash.

[peterbarker]

Looks right.

Was it causing an issue?

Marked for DevCall to see what people think of treewide: as a concept...

[sorki]

On NixOS there's no /bin/bash 😄 There's only /bin/sh and /usr/bin/env (only two binaries in these directories).

This allows using scripts and building everything on NixOS - If there's interest I can publish a follow-up PRs with Nix files that allow quickly setting up a development shell or performing a full build without having to fiddle with dependencies.

[peterbarker]

Merged, thanks!