fix: Change bash shebang to use env

[awebeer256]

This is required for NixOS, where /usr/bin contains nothing but env. The sh shebang does not need to be changed, since sh is still in /bin on NixOS (in fact, it's the ONLY thing in that directory).

[bigpod98]

Just asking for clarification but what does NixOS have to do with ublue nvidia images and generating akmods keys for them?

[awebeer256]

Per the readme:

If you are forking this repo, then you should add a private key to the repository secrets:

$ ./generate-akmod-key
$ gh secret set AKMOD_PRIVKEY < certs/private_key.priv.prod
$ cp certs/public_key.der.prod certs/public_key.der

This happens on whatever machine you're currently using, before you install the image (before you can build it, in fact). In my case, that was a NixOS machine.