Improve CI times

[tedinski]

Description of changes:

1. Do not needlessly re-trigger the standard regression suite on "tag". Only on push of commits to main. This eliminates some wait time in the release process from being throttled on too many CI tasks running at once.
2. (Minor change to setting RUST_BACKTRACE)
3. Disable the stdlib test on macos for the moment. When someone has time to speed this up, we should re-enable it. Tracked in Re-enable stdlib codegen test on macos #1578

Resolved issues:

Call-outs:

Testing:

• How is this change tested? ci

• Is this a refactor change? no

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[tedinski]

Well, it cut us from 38 minutes to 31 minutes. I'm baffled by why macos is so slow in CI.

[zhassan-aws]

[zhassan-aws]

This run is from a while back, but notice how every single step seems to be slower on macos-11.