Disable install test by default - only run manually

The install test builds too many dependencies from source (60+ min):
- cmake: 8+ minutes
- openmpi: 4+ minutes
- Binary cache doesn't have matching architectures
- Times out the 45 minute limit

Now only runs on manual workflow_dispatch trigger.

Fast tests still run on every commit:
- Lint & Style Check (~2 min)
- Spec Concretization (~2 min)

These validate the package definition is correct.
The install test can be run manually before Spack PR submission.

Author: sbryngelson

.github/workflows/spack.yml

@@ -112,8 +112,9 @@ jobs:
 
   test-install:
     name: Test Installation & Execution
-    # Only run on pull requests or manual workflow dispatch to save CI time
-    if: github.event_name == 'pull_request' || github.event_name == 'workflow_dispatch'
+    # Disabled by default - takes too long (60+ min building from source)
+    # Enable manually via workflow_dispatch when needed
+    if: github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout MFC