ci: update to latest intel osx runner and download 10.15 SDK

[beckermr]

Description

This PR updates the CI to the latest intel osx runner and to download the 10.15 SDK.

I have preserved the older option of directly using the system SDK via xcode. This should make it easier for folks to work locally in some cases without doing their own SDK download.

Resolves #5378

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[codspeed-hq]

CodSpeed Performance Report

Merging #5387 will not alter performance

_{Comparing beckermr:mac12 (cbe5d3a) with main (c7a1e9b)}

Summary

✅ 3 untouched benchmarks

[kenodegard]

I also started on this in #5379

While the SDK downloads are working I haven't quite gotten the tests to pass again

[beckermr]

OK @kenodegard this one is ready for review!

This PR has the most minimal changes I could make to get the test suite to pass on the macos-13 runners.

Some notes and comments:

• Your approach in Create fixture to fetch MacOSX SDKs #5379 for the SDKs is more complete for sure. However, it does couple code in the test suite (the fixture) with the locally cached directory on the runners (~/macosx_sdks).
• I have opted here to keep all of the SDK handling in GHA and have the test suite use w/e sysroot the user presents to it, either through xcode or the CONDA_BUILD_SYSROOT environment variable.
• The approach here may help folks who use the test suite locally in that they can easily use w/e sysroot they like for testing by setting the CONDA_BUILD_SYSROOT.
• Intel mac hardware is going away and there are lots of tests that have to be run on osx-64 due to packages not be available on osx-arm64. I documented part of this in issue tests use packages not on osx-arm64 #5388. We have time to address this one too.

[beckermr]

@conda/builds-tools @kenodegard The CI here will break starting TOMORROW if we do not merge something to remove macos-11. Shall we merge this one?

[kenodegard]

@beckermr thanks for working on this!