Improve deployment for test builds

As we've had a few issues with deployments recently, I thought it might
be useful to spot these issues earlier if we get each PR to also run the
preliminary steps of the build process. This should also stop regression
like issues (e.g. newer versions of poetry breaks our build process).

.github/workflows/pre-release.yml

@@ -3,11 +3,13 @@ name: Deploy to test.pypi.org
 on:
   push:
     branches: [master]
+  pull_request:
+    branches: [master]
 
 jobs:
-  deploy:
+  # Build the python package even on PRs to ensure we're able to build the package properly.
+  build:
     runs-on: ubuntu-20.04
-    steps:
       - uses: actions/checkout@v4
 
       - name: Set up Python
@@ -33,14 +35,48 @@ jobs:
           # latest GH release.
           poetry version $LATEST_RELEASE
           poetry version prerelease  # Using `prerelease` rather than `prepatch` due to a bug in Poetry (latest checked 1.8.1 - https://github.com/python-poetry/poetry/issues/879)
+          poetry build
+
+      - name: Save build
+        uses: actions/cache/save@v4
+        id: build-cache
+        with:
+          path: |
+            dist/
+          key: cache-${{ github.run_id }}-${{ github.run_attempt }}
+
+  deploy:
+    runs-on: ubuntu-20.04
+    needs: build
+    if: github.ref == 'refs/heads/master'
+    steps:
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.x"
+          cache: pip
+
+      - name: Install dependencies
+        run: python -m pip install --upgrade pip poetry
+
+      - name: Get build
+        uses: actions/cache/restore@v4
+        with:
+          path: |
+            dist/
+          key: cache-${{ github.run_id }}-${{ github.run_attempt }}
+
+      - name: Deploy to testpypi.org
+        run: |
           poetry config repositories.test_pypi https://test.pypi.org/legacy/
-          poetry publish --build -r test_pypi --username __token__ --password ${{ secrets.TEST_PYPI_TOKEN }} || true
+          poetry publish -r test_pypi --username __token__ --password ${{ secrets.TEST_PYPI_TOKEN }} || true
 
   update_release_draft:
     permissions:
       contents: write
       pull-requests: read
     runs-on: ubuntu-latest
+    needs: deploy
     steps:
       - uses: release-drafter/release-drafter@v5
         with: