Don't push to ghcr on PR builds, after all

Turns out external contributors aren't allowed to push to GHCR, so
their PRs don't get successful statuses on the docker action. That's
annoying, so let's instead just build the image and not push it on
PRs.

Author: antifuchs

dev/flake-part.nix

@@ -54,7 +54,7 @@
           inherit branch;
 
           github = {
-            enable = true;
+            enable = !isPR;
             token = "$GH_TOKEN";
           };