Revert "Run tests sequentially: cargo test -j 1"

It does not work:

* https://github.com/nix-rust/nix/actions/runs/19062321851/job/54444625438?pr=2689
* https://github.com/nix-rust/nix/actions/runs/19062321851/job/54444910017?pr=2689

Author: SteveLauC

.github/actions/test/action.yml

@@ -34,4 +34,4 @@ runs:
 
     - name: test
       shell: bash
-      run: ${{ inputs.SUDO }} $(which ${{ inputs.TOOL }}) test --target ${{ inputs.TARGET }} -j 1
+      run: ${{ inputs.SUDO }} $(which ${{ inputs.TOOL }}) test --target ${{ inputs.TARGET }}