refactor(nix): use date for version

Helps avoid needless rebuilds where only the revision changed. Also
better conforms to Nixpkgs' version standards

Signed-off-by: Seth Flynn <getchoo@tuta.io>
(cherry picked from commit e9cac2e0e37c4531f662545bdc45b72499654642)

nix/unwrapped.nix

@@ -24,9 +24,28 @@ assert lib.assertMsg (
   gamemodeSupport -> stdenv.hostPlatform.isLinux
 ) "gamemodeSupport is only available on Linux.";
 
+let
+  date =
+    let
+      # YYYYMMDD
+      date' = lib.substring 0 8 self.lastModifiedDate;
+      year = lib.substring 0 4 date';
+      month = lib.substring 4 2 date';
+      date = lib.substring 6 2 date';
+    in
+    if (self ? "lastModifiedDate") then
+      lib.concatStringsSep "-" [
+        year
+        month
+        date
+      ]
+    else
+      "unknown";
+in
+
 stdenv.mkDerivation {
   pname = "prismlauncher-unwrapped";
-  version = self.shortRev or self.dirtyShortRev or "unknown";
+  version = "10.0-unstable-${date}";
 
   src = lib.fileset.toSource {
     root = ../.;