gh-36016: CI Build & Test: Fix test errors involving optional packages coxeter3, ...
    
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->

<!-- Why is this change required? What problem does it solve? -->
These errors have started to show up after #35661 was merged.

Optional packages such as `coxeter3` are installed as a side effect of
testing distributions such as sagemath-coxeter.
Hence the doctests marked `# optional - coxeter3` are activated.
Here we ensure that the corresponding extension modules are also
installed in the venv where the doctests run.

<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #36016
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

Author: Release Manager

.github/workflows/build.yml

@@ -103,12 +103,24 @@ jobs:
         run: pyright
         working-directory: ./worktree-image
 
-      - name: Build (fallback to non-incremental)
-        id: build
+      - name: Clean (fallback to non-incremental)
+        id: clean
         if: always() && steps.worktree.outcome == 'success' && steps.incremental.outcome != 'success'
         run: |
           set -ex
-          ./bootstrap && make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status && make build
+          ./bootstrap && make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status
+        working-directory: ./worktree-image
+        env:
+          MAKE: make -j2
+          SAGE_NUM_THREADS: 2
+
+      - name: Build
+        # This step is needed because building the modularized distributions installs some optional packages,
+        # so the editable install of sagelib needs to build the corresponding optional extension modules.
+        id: build
+        if: always() && (steps.incremental.outcome == 'success' || steps.clean.outcome == 'success')
+        run: |
+          make build
         working-directory: ./worktree-image
         env:
           MAKE: make -j2
@@ -125,22 +137,22 @@ jobs:
           COLUMNS: 120
 
       - name: Test all files (sage -t --all --long)
-        if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
+        if: always() && steps.build.outcome == 'success'
         run: |
           ../sage -python -m pip install coverage
           ../sage -python -m coverage run ./bin/sage-runtests --all --long -p2 --random-seed=286735480429121101562228604801325644303
         working-directory: ./worktree-image/src
 
       - name: Prepare coverage results
-        if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
+        if: always() && steps.build.outcome == 'success'
         run: |
           ./venv/bin/python3 -m coverage combine src/.coverage/
           ./venv/bin/python3 -m coverage xml
           find . -name *coverage*
         working-directory: ./worktree-image
 
       - name: Upload coverage to codecov
-        if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
+        if: always() && steps.build.outcome == 'success'
         uses: codecov/codecov-action@v3
         with:
           files: ./worktree-image/coverage.xml