gh-36473: CI docbuild: Do not rebuild sagelib from scratch
    
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Reworking #35652 as discussed in
#36469 (comment)

Also applying the changes made in #35866 (cosmetic improvements to the
Actions logs of doc-build.yml) to doc-build-pdf.yml

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URL: #36473
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

Author: Release Manager

.github/workflows/doc-build-pdf.yml

@@ -89,27 +89,28 @@ jobs:
           ./bootstrap && make build
         working-directory: ./worktree-image
         env:
-          MAKE: make -j2
+          MAKE: make -j2 --output-sync=recurse
           SAGE_NUM_THREADS: 2
 
       - name: Build (fallback to non-incremental)
         id: build
         if: always() && steps.worktree.outcome == 'success' && steps.incremental.outcome != 'success'
         run: |
           set -ex
-          make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status && make build
+          make sagelib-clean && git clean -fx src/sage && ./config.status && make build
         working-directory: ./worktree-image
         env:
-          MAKE: make -j2
+          MAKE: make -j2 --output-sync=recurse
           SAGE_NUM_THREADS: 2
 
       - name: Build docs (PDF)
         id: docbuild
         if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
-        run: make build V=0 && make doc-pdf
+        run: |
+          make doc-clean doc-uninstall; make doc-pdf
         working-directory: ./worktree-image
         env:
-          MAKE: make -j2
+          MAKE: make -j2 --output-sync=recurse
           SAGE_NUM_THREADS: 2
 
       - name: Copy docs

.github/workflows/doc-build.yml

@@ -88,7 +88,7 @@ jobs:
         if: always() && steps.worktree.outcome == 'success' && steps.incremental.outcome != 'success'
         run: |
           set -ex
-          make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status && make build
+          make sagelib-clean && git clean -fx src/sage && ./config.status && make build
         working-directory: ./worktree-image
         env:
           MAKE: make -j2 --output-sync=recurse
@@ -103,7 +103,7 @@ jobs:
           set -ex
           export SAGE_USE_CDNS=yes
           mv /sage/local/share/doc/sage/html/en/.git /sage/.git-doc
-          make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage
+          make doc-clean doc-uninstall
           mkdir -p /sage/local/share/doc/sage/html/en/ && mv /sage/.git-doc /sage/local/share/doc/sage/html/en/.git
           ./config.status && make doc-html
         working-directory: ./worktree-image