github workflows: Stick with which instead of command -v

... because even though it looks like a subshell, it apparently is not
evaluated through a shell but passed right into exec.

.github/workflows/release-test.yml

@@ -150,7 +150,7 @@ jobs:
           TTN_DEV_ID="riot_lorawan_1" \
           TTN_DEV_ID_ABP="riot_lorawan_1_abp" \
           RIOTBASE=${RIOTBASE} \
-          $(command -v tox) -e test -- ${TOX_ARGS} \
+          $(which tox) -e test -- ${TOX_ARGS} \
             ${K} "${{ github.event.inputs.filter }}" -m "${{ matrix.pytest_mark }}"
     - name: junit2html and XML deploy
       if: always()