use a clan-core snapshots for ci tests

We currently have to re-run our integration tests a lot because they are
depending on the whole repository. This pull request changes locks the
clan-core used for vm tests. This has the caveat that we might not run
the latest NixOS machine of our profiles. On the upside we can test
behaviour against an older clan-core version and capture breakages and
make it backwards compatible. If we actually want to test the latest
version, the PR that changes the exposed flake api, could also bump the
clan-core snapshot.

flakeModules/demo_iso.nix

@@ -37,7 +37,7 @@ let
       done
       if ! test -e ~/clan-core; then
         # git clone https://git.clan.lol/clan/clan-core.git ~/clan-core
-        cp -rv ${self} clan-core
+        cp -rv ${self.checks.x86_64-linux.clan-core-for-checks} clan-core
       fi
       cd clan-core
       clan machines morph demo-template --i-will-be-fired-for-using-this