firmwareLinuxNonfree: fix fetching from git (#116306)

Since https://github.com/NixOS/nixpkgs/pull/104714 using a tag as
the `rev` parameter to `fetchgit` is no longer reliable, so spell
out the rev.

Fixes #114439

pkgs/os-specific/linux/firmware/firmware-linux-nonfree/default.nix

···
 
   src = fetchgit {
     url = "https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git";
-    rev = lib.replaceStrings ["-"] [""] version;
+    rev = "refs/tags/" + lib.replaceStrings ["-"] [""] version;
     sha256 = "0c85cd659312isfz1r87qswsgfhy0rljagcwspnvjljqrh9bsgzq";
   };