Prevent @actions/core from writing to files in unittests

Clearing the github defined variables makes @actions/core print to
stdout instead of writing to the files, which allows the tests to
mock other platforms without the fear of paths breaking.

Signed-off-by: Andrei Horodniceanu <[email protected]>

.github/workflows/unittests.yml

@@ -30,4 +30,4 @@ jobs:
         run: |
           set -euxo pipefail
           npm ci
-          npm test
+          GITHUB_ENV= GITHUB_PATH= npm test