ci: Fix syntax error in workflow file

[perlpunk]

See also #588

[nitzmahone]

@perlpunk We actually need to re-add the rest of the 3.8 x86_64 job for Windows where that line came from- I got over-zealous with the "scissors" when I was removing EOL Python matrix entries, but since that's one of the commented out parts of the CI copy of the workflow, it didn't break. I can just add a commit to this PR if you'd rather, or whatever...

Bigger picture: I've got a working dynamic matrix generator that eliminates the need for the two copies of the workflow altogether. With that, we just use whatever criteria we want to dynamically omit jobs that aren't applicable/possible in a given context (eg, the jobs that require the self-hosted Apple Silicon Mac runners and the slow/emulated jobs). It's a major bummer that GHA still doesn't provide any supported mechanism for doing this (and also to be able to represent the omitted jobs), but at least we have a solution now that can work.

[perlpunk]

Well, it broke for me: https://github.com/perlpunk/pyyaml/actions/runs/6835080303
I saw Invalid workflow file: .github/workflows/manual_artifact_build.yaml#L353.
But if you want to fix it differently, I'll just close this.