Fix various issues in unstable CI

[djhoese]

cftime's master branch should support numpy 2 now, but from what I could tell our CI still wasn't working. My best guess is the conda-installed version is getting used before the pip one. This PR first removes the conda installed one in unstable CI before installing from source.

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.94%. Comparing base (7f9cea5) to head (82899d9).
Report is 643 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2784    +/-   ##
========================================
  Coverage   95.93%   95.94%            
========================================
  Files         377      379     +2     
  Lines       53559    53673   +114     
========================================
+ Hits        51381    51494   +113     
- Misses       2178     2179     +1     

Flag	Coverage Δ
behaviourtests	4.09% <ø> (-0.01%)	⬇️
unittests	96.03% <ø> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 8756273146

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 8732515933:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[djhoese]

@mraspaud Ok I've got CI working in unstable as far as numpy 2 goes and what we have control over. The remaining things are:

1. The xarray dtype/scalar handling that I have a PR for in xarray.
2. Things based on numba fail since numba does not support numpy 2 yet.

I think this is ready for merge.

[mraspaud]

LGTM