tox,workflow: pass tests to run via tox {posargs} instead of env

Using an environ for passing the tests is a bit of a headache when
it comes to quoting which is important when trying to write something
like: `-k "not test_stages.py"`.

I (personally) also find it slightly nicer/more intuitive to be able to do:
```
$ tox -e py36 -- ./test/mod
```
compared to
```
$ TEST_CATEGORY="./test/mod" tox -e py36
```

.github/workflows/test.yml

@@ -54,10 +54,9 @@ jobs:
           # it to vfs for the local storage skopeo stage test.
           sed -i 's/overlay/vfs/g' /usr/share/containers/storage.conf  # default system config
           sed -i 's/overlay/vfs/g' /etc/containers/storage.conf || true  # potential overrides
-          TEST_CATEGORY="${{ matrix.test }}" \
           TEST_WORKERS="${{ matrix.env_TEST_WORKERS || env.TEST_WORKERS }}" \
           OSBUILD_TEST_STORE="${{ env.OSBUILD_TEST_STORE }}" \
-          tox -e "${{ matrix.environment }}"
+          tox -e "${{ matrix.environment }}" -- ${{ matrix.test }}
 
   v1_manifests:
     name: "Assembler test (legacy)"
@@ -73,7 +72,6 @@ jobs:
         with:
           image: ghcr.io/osbuild/osbuild-ci:latest-202308241910
           run: |
-            TEST_CATEGORY="test.run.test_assemblers" \
             TEST_WORKERS="${{ env.TEST_WORKERS }}" \
             OSBUILD_TEST_STORE="${{ env.OSBUILD_TEST_STORE }}" \
-            tox -e "py36"
+            tox -e "py36" -- test.run.test_assemblers

tox.ini

@@ -34,7 +34,7 @@ passenv =
     TEST_CATEGORY
 
 commands =
-    bash -c 'python -m pytest -v --pyargs --rootdir=. {env:TEST_CATEGORY} {env:TEST_WORKERS}'
+    python -m pytest -v --pyargs --rootdir=. {posargs}
 
 allowlist_externals =
     bash