TST: temp pin macos x86 runners

[neutrinoceros]

Description

context: https://github.blog/changelog/2024-04-01-macos-14-sonoma-is-generally-available-and-the-latest-macos-runner-image/

As can be seen in example logs, our CI was automatically migrated from macos-12 to macos-14 images. This migration also involves a change in hardware arch from x86 to arm64, so regular CI is now hitting #16319 and #16320
This PR pins images to macos-12 (x86) as a temp measure while these issues are being figured out.
I'll do the migration explicitly in #16321 when we stabilize it.

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[pllim]

LGTM as long as CI is green. Thanks!