Remove hardcoded bash location in tools/preprocess.sh

[CharlesAverill]

tools/preprocess.sh contains a hardcoded shebang to /bin/bash that ignores the SHELL variable in Makefile. This breaks the build process for NixOS (or anyone else who doesn't have a /bin/bash) with a cryptic error. I've removed the shebang.

[CharlesAverill]

That does complicate things... could check $SHELL for something that ends in /bash?

[cadmic]

would #!/usr/bin/env bash work for you?

[CharlesAverill]

Yes, that does work!