CI: Fix using `check_source` flags as bool by webknjaz · Pull Request #121848 · python/cpython

Previously, those flags would sometimes end up having empty string
values, which tends to break evaluating them as JSON. This patch adds
`false` fallbacks to all such outputs.

This allows feeding them to `fromJSON()` without a fear of them
causing surprising internal behaviors in the GitHub Actions CI/CD
workflows platform itself [[1]]. The behavior observed was that
some skipped jobs wouldn't show up in the workflow sidebar view at
all, would display in the graph view as `Waiting for pending jobs`
and in the `${{ needs }}` context, they would have a
`result: failure` entry [[2]].

This should help make PRs like python#121831 mergeable again.

[1]: python#121766 (comment)
[2]: https://github.com/python/cpython/actions/runs/9950331379/job/27501637459?pr=121831#step:2:244

This updates it to avoid referencing to `no-GIL` in favor of the
official term `free-threading`.

(cherry picked from commit a0b205b)

Co-authored-by: Sviatoslav Sydorenko (Святослав Сидоренко) <wk@sydorenko.org.ua>

(cherry picked from commit a0b205b)

Co-authored-by: Sviatoslav Sydorenko (Святослав Сидоренко) <wk@sydorenko.org.ua>