gn: make src version overridable

Co-authored-by: emilylange <[e-mail]>
Signed-off-by: Marcin Serwin <[e-mail]>

pkgs/by-name/gn/gn/package.nix

···
   cctools,
   ninja,
   python3,
+
+  # Note: Please use the recommended version for Chromium stable, i.e. from
+  # <nixpkgs>/pkgs/applications/networking/browsers/chromium/info.json
+  version ?
+    # This is a workaround for update-source-version to be able to update this
+    let
+      _version = "0-unstable-2025-04-28";
+    in
+    _version,
+  rev ? "85cc21e94af590a267c1c7a47020d9b420f8a033",
+  hash ? "sha256-SYpWYvGGNLMZPR1LdKKgubbv+4QcxRw8EYp7cNI5wQQ=",
 }:
 
 stdenv.mkDerivation {
   pname = "gn";
-  version = "0-unstable-2025-04-28";
+  inherit version;
 
   src = fetchgit {
     # Note: The TAR-Archives (+archive/${rev}.tar.gz) are not deterministic!
     url = "https://gn.googlesource.com/gn";
-    # Note: Please use the recommended version for Chromium stable, i.e. from
-    # <nixpkgs>/pkgs/applications/networking/browsers/chromium/info.json
-    rev = "85cc21e94af590a267c1c7a47020d9b420f8a033";
-    hash = "sha256-SYpWYvGGNLMZPR1LdKKgubbv+4QcxRw8EYp7cNI5wQQ=";
+    inherit rev hash;
     leaveDotGit = true;
     deepClone = true;
     postFetch = ''

pkgs/by-name/gn/gn/update.sh

···
 commit_date=$(TZ= date --date "$commit_time" --iso-8601)
 version="0-unstable-$commit_date"
 
-update-source-version --rev="$rev" --version-key="version" "gn" "$version"
+update-source-version --rev="$rev" --version-key="_version" "gn" "$version"