Auto merge of #29201 - delan:ci-mac-wpt, r=jdm

CI: run WPT tests on macOS

This patch uncomments and fixes the mac-wpt job in the main workflow, with a few changes:

* we downgrade python3 and pip3 to 3.9, because inspect.getargspec was [removed in 3.11](https://docs.python.org/3/whatsnew/3.11.html#removed)
    * we should apparently move to [inspect.getfullargspec](https://docs.python.org/3/library/inspect.html#inspect.getfullargspec)
* we download the real Servo artifact, rather than the fake Servo for testing

We also align the rest of the job (other than `--timeout-multiplier`) to be as close as possible to linux-wpt, based on the assumption that linux-wpt is more up to date. This includes:

* setting strategy.fail-fast to false
* adding a failure() condition to the “Archive logs” step
* adding the chunk id to log filenames

There are some failing WPT tests, so for now the job is not required to succeed.

Example run: <https://github.com/delan/servo/actions/runs/3837530905>

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- [ ] ~~`./mach build -d` does not report any errors~~
- [ ] ~~`./mach test-tidy` does not report any errors~~
- [x] These changes fix #28284 (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they affect the CI configuration

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