CI: Skip install_solidus check on non-current version

The installer always uses the current released minor version
(4.2 time writing this), but the check runs on all pipeline
runs, wasting CI time and precious resources. Also it fails
anyway, because of conflicting dependencies.

So, we only run the check on main and latest minor now.

.circleci/config.yml

@@ -266,6 +266,11 @@ jobs:
     executor:
       name: sqlite
       ruby: "3.0"
+    when:
+      condition:
+        or:
+          - equal: [main, << pipeline.git.branch >>]
+          - equal: [v4.2, << pipeline.git.branch >>]
     steps:
       - checkout
       - libvips