nixos/udev: allow /bin/sh in udev rules

Using `/bin/sh` in udev rules is fine (as it's guaranteed to point to a
(bash) shell on NixOS), and actually is better than hardcoding absolute
paths, at least in cases where these rules are also added to the
(systemd-based) initrd (via boot.initrd.services.udev.rules).

To allow this, we need to update the check routine that assembles the
list of files needing fixup, to explicitly exclude `/bin/sh` occurences.

To do this, we convert the pattern to a PCRE regex (which requires "/"
to be escaped), and add `(?!/bin/sh\b)` as a negative lookahead.

This subsequently allows udev rules to (start using) `/bin/sh` again, so
they'll work in-initrd.

nixos/modules/services/hardware/udev.nix

@@ -112,7 +112,8 @@ let
       echo "OK"
 
       filesToFixup="$(for i in "$out"/*; do
-        grep -l '\B\(/usr\)\?/s\?bin' "$i" || :
+        # list all files referring to (/usr)/bin paths, but allow references to /bin/sh.
+        grep -P -l '\B(?!\/bin\/sh\b)(\/usr)?\/bin(?:\/.*)?' "$i" || :
       done)"
 
       if [ -n "$filesToFixup" ]; then