Do just 1 "parallel" job

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -875,7 +875,7 @@ jobs:
           make CXX=clcache BUILD_ENV=MSVC -C unit test TAGS="[z3]"
           make CXX=clcache BUILD_ENV=MSVC -C jbmc/unit test
       - name: Run CBMC regression tests
-        run: make CXX=clcache BUILD_ENV=MSVC -C regression test-parallel JOBS=${{env.windows-vcpus}}
+        run: make CXX=clcache BUILD_ENV=MSVC -C regression test-parallel JOBS=1
 
   # This job takes approximately 7 to 32 minutes
   windows-msi-package: