test.yml: get back to a single pytest worker

Suspecting race conditions here:

```
Build failed: ['forge', 'build', '--ast', '--root', 'tests/regression', '--extra-output', 'storageLayout', 'metadata']
----------------------------- Captured stderr call -----------------------------
Failed to install solc 0.8.26: Text file busy (os error 26)
Error:
Text file busy (os error 26)
```

.github/workflows/test.yml

@@ -53,4 +53,4 @@ jobs:
         run: python -m pip install -e .
 
       - name: Run pytest
-        run: pytest -n 4 -v -k "not long and not ffi" --ignore=tests/lib --halmos-options="--debug -st ${{ matrix.parallel }} --storage-layout ${{ matrix.storage-layout }} --solver-timeout-assertion 0 ${{ inputs.halmos-options }}" ${{ inputs.pytest-options }}
+        run: pytest -n 1 -v -k "not long and not ffi" --ignore=tests/lib --halmos-options="--debug -st ${{ matrix.parallel }} --storage-layout ${{ matrix.storage-layout }} --solver-timeout-assertion 0 ${{ inputs.halmos-options }}" ${{ inputs.pytest-options }}