gh-36694: CI conda: On `pull_request`, only run 1 macOS job and 1 Linux job
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as discussed in
https://github.com/sagemath/sage/pull/36678#issuecomment-1806192342,
https://github.com/sagemath/sage/pull/36616#issuecomment-1811508709
On pushes to a tag, all 3 Linux and 3 macOS jobs are still run, as
demonstrated in https://github.com/mkoeppe/sage/actions/runs/6829619271
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After merging, the branch protection rule https://github.com/sagemath/sa
ge/settings/branch_protection_rules/33965567 will need to be updated. It
controls what workflows show as "Required" in the PR Checks.
### :memo: Checklist
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/36694
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Tobias Diez
