Reduce REFERENCE_TESTS_PARALLELISM

This number is set too high for the given runner.  I've reduced it to
just `1` for now to try and get things working again.  Subsequently we
can try incrementing this to find the sweet spot.

.github/workflows/reference-blockchain-tests.yml

@@ -66,7 +66,7 @@ jobs:
         run: ./gradlew referenceBlockchainTests -x spotlessCheck
         timeout-minutes: 180
         env:
-          REFERENCE_TESTS_PARALLELISM: 7
+          REFERENCE_TESTS_PARALLELISM: 1
           JAVA_OPTS: -Dorg.gradle.daemon=false
           CORSET_FLAGS: disable
           GOCORSET_FLAGS: -wd --ansi-escapes=false --report --air