linux-firmware: use commit instead of tag

[raboof]

Motivation for this change

Since the repo appears to be somewhat corrupted and switching to the tag
from nix-build does not currently work consistently.

#114439

Things done

Tested with nix-build and the --option substitute false option to
verify nix isn't using the sha256. Indeed this failed without the change
and succeeded after the change.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/what-are-your-goals-for-21-05/11559/4

[raboof]

Result of nixpkgs-review pr 114588 run on x86_64-linux 1

[primeos]

Let's close this in favor of #116306 :)