Revert "treewide: replace rev with tag"

This reverts commit 65a333600d.

This wasn't tested for correctness with something like fodwatch [0],
and should not have been (self-)merged so quickly, especially without
further review.

It also resulted in the breakage of at least one package [1] (and that's
the one we know of and was caught).

A few packages that were updated in between this commit and this revert
were not reverted back to using `rev`, but other than that, this is a
1:1 revert.

[0]: https://codeberg.org/raphaelr/fodwatch
[1]: https://github.com/NixOS/nixpkgs/pull/396904 / 758551e458

nixos/tests/dokuwiki.nix

@@ -9,7 +9,7 @@ let
     src = pkgs.fetchFromGitHub {
       owner = "giterlizzi";
       repo = "dokuwiki-template-bootstrap3";
-      tag = "v${version}";
+      rev = "v${version}";
       hash = "sha256-B3Yd4lxdwqfCnfmZdp+i/Mzwn/aEuZ0ovagDxuR6lxo=";
     };
     installPhase = "mkdir -p $out; cp -R * $out/";