[CI] Condition adding the SSH key to main branch

Secrets are not available to branches from forked repos, so the SSH
setup step has to be skipped unless the main branch is being used.

Fixes #882

.github/workflows/main.yml

@@ -138,6 +138,7 @@ jobs:
       # machine that has previously logged in to cantera.org and trusts
       # that it logged in to the right machine
       - name: Set up SSH key and host for deploy
+        if: github.event_name == 'push' && github.repository_owner == 'Cantera' && github.ref == 'master'
         uses: shimataro/ssh-key-action@v2
         with:
           key: ${{ secrets.CTDEPLOY_KEY }}