Fix syntax for GitHub workspace

Author: wimpyprogrammer

.github/workflows/tests.yml

@@ -38,7 +38,7 @@ jobs:
       - run: yarn run test
 
       - run: npx testpack-cli --keep=@types/*,ts-jest,typescript jest.config.js tsconfig.test.json src/e2e.spec.ts
-        working-directory: $GITHUB_WORKSPACE/packages/regex-to-strings
+        working-directory: ${{ github.workspace }}/packages/regex-to-strings
 
       - name: Upload test coverage report to Codecov
         uses: codecov/codecov-action@v2.1.0