workflows: Safer variable access

This does not seem like a serious issue but
* zizmor complains about it
* not assigning GHA variables in middle of bash scripts is a good idea in general.

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

.github/workflows/_test.yml

@@ -83,7 +83,9 @@ jobs:
           fi
 
       - name: Run tox
-        run: tox -e ${{ matrix.toxenv }}
+        env:
+          TOXENV: ${{ matrix.toxenv }}
+        run: tox
 
   summary:
     needs: test