`file` is used by the perl script.
```
sub _get_file_type($) {
my $file=shift;
open(FILE, '-|') # handle all filenames safely
|| exec('file', $file)
|| die "can't exec file: $!";
my $type=<FILE>;
close FILE;
return $type;
}
```
This script is very handy to run within a `nix-build` context,
specifically during the fixupPhase.
Unfortunately, file is not propagated, and does not exist causing the
build to fail. Fix it by adding it.
Co-authored-by: Jonathan Ringer <jonringer@users.noreply.github.com>
