Only build and test with container on upstream (#1817)

As it stands, since we are evaluating using the Docker Hub registry
that is not public to facilitate NIST Team development to be more
efficient for local and remote testing. As this container may not
exist at first commit of branch, disable on fork-based PRs for now.

.github/workflows/xspec-tests.yml

@@ -25,6 +25,7 @@ on:
   workflow_call: {}
 jobs:
   xspec-tests:
+    if: github.repository == 'usnistgov/OSCAL' || github.event.pull_request.head.repo.name == github.repository
     name: Run XSpec tests
     runs-on: ubuntu-22.04
     steps: