fix shebang in bin/build.sh

[sorki]

Using env is a more portable approach.

[haslinghuis]

Good find. But not sure if we need anything to fix.

https://stackoverflow.com/questions/21612980/why-is-usr-bin-env-bash-superior-to-bin-bash

[sorki]

I found it by cloning the repo and trying to run make to test vs betaflight 4.3. My distro (NixOS) doesn't have /bin/bash (not even /usr/bin/bash) so we need to use env. The change is pretty harmless and should work everywhere.