ci: cache pip before install and fix cache key OS expansion

Move the actions/cache step ahead of the dependency install so a
warm cache actually accelerates the install, and replace the
unexpanded $RUNNER_OS shell variable with the ${{ runner.os }}
Actions expression in the cache key and restore-keys.

Fixes #8

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-and-publish.yaml

@@ -45,16 +45,16 @@ jobs:
         with:
           python-version: 3.12
 
-      - name: Install test dependencies
-        run: python -m pip install --upgrade pip && pip install -r requirements-dev.txt
-
       - name: Cache pip dependencies
         uses: actions/cache@v4
         with:
           path: ~/.cache/pip
-          key: "$RUNNER_OS-pip-${{ hashFiles('requirements-dev.txt') }}"
+          key: "${{ runner.os }}-pip-${{ hashFiles('requirements-dev.txt') }}"
           restore-keys: |
-            $RUNNER_OS-pip-
+            ${{ runner.os }}-pip-
+
+      - name: Install test dependencies
+        run: python -m pip install --upgrade pip && pip install -r requirements-dev.txt
 
       - name: Run unit tests
         run: python -m pytest -q