CI: move conda build test to seperate job

[fangchenli]

Reference Issues or PRs

xref: #1128

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

Any other comments?

Since nebari does not have non-python extensions, its wheels, and sdists are identical for different Python versions. So it's not necessary to run conda build test in all unit tests jobs. In fact, most of the time spent in unit tests is from the conda build test.

In this PR, I moved the conda build test to a separate job. Combine with #2071, the runtime of unit tests jobs reduced from 30+min to ~2min.

Note: merge before #2071.

[iameskild]

This makes sense. Initially I wanted it part of the pytest suite but I now end up skipping that test unless I explicit update the package dependencies.

Thanks for all the recent clean up 💯