Change shebang to use /usr/bin/env

[asymmetric]

Makes the script more portable to systems where perl is not in /usr/bin, e.g. NixOS.

[digint]

If changing to /usr/bin/env perl, we would require to have a valid PATH environment, pointing to wherever perl is located.
This is not always the case, e.g. quite common within initrd.

Not sure if this would be the right thing to do anyways, as btrbk wants to be run by the "system perl", which is expected to be in /usr/bin (i know, some distros think differently, that's what makes it hard to find a decent compromise)