gh-35652: CI Build & Test: First build incrementally and test changed files only
    
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### 📚 Description

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This gives a faster turnaround for errors in the committed changes.
When the incremental build (or the test) fails, we clean thoroughly and
build sagelib from scratch (= what happened before this PR.)

Changing the docbuild CI in the same way.

The full doctests are now run with `--long`.

Also:
- Fixes `sage -t --new` so it works in a git worktree and respects file-
level annotations `# sage.doctest: optional - ...`
- Resolves #35732

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- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

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- Depends on #35751 (merged here)
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URL: #35652
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe, Tobias Diez