CI: Add a CI job that runs the post-build tests on the versions.json that is currently in S3

(cherry picked from commit 3020d7f)

Author: DilumAluthge

.github/workflows/CI.yml

@@ -136,4 +136,4 @@ jobs:
       - name: Download the current versions.json from S3
         run: curl -LO https://julialang-s3.julialang.org/bin/versions.json
       - name: Run the post-build tests on the versions.json that we downloaded from S3
-        run: julia --project test/more_tests.jl versions.json
+        run: julia --project test/post_build.jl versions.json full