Improve GHA conda setup performance

[jsiirola]

Fixes # .

Summary/Motivation:

This PR reworks how the GHA jobs configure conda environments, with an eye to reducing the overall GHA test time. This makes two significant changes:

• use a consistent set of conda channels instead of specifying a single channel for each conda install command. This prevents unnecessary rework (e.g., installing a package and then later upgrading it from a different channel)
• use mamba to solve the environment, if possible. This improved solver can be significantly more efficient than the default conda solver.

Preliminary testing shaves ~20 minutes off the win/3.9 job and more than an hour off the problematic win/3.11 job.

Changes proposed in this PR:

• use mamba to solve the environment, if possible
• set a global list of channels (with consistent priority)

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

Base: 87.06% // Head: 87.04% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (64b0e48) compared to base (1c8c5ab).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2701      +/-   ##
==========================================
- Coverage   87.06%   87.04%   -0.02%     
==========================================
  Files         757      757              
  Lines       84596    84596              
==========================================
- Hits        73652    73639      -13     
- Misses      10944    10957      +13     

Flag	Coverage Δ
linux	84.45% <ø> (ø)
osx	74.50% <ø> (ø)
other	84.63% <ø> (ø)
win	81.78% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/contrib/pyros/pyros_algorithm_methods.py	93.37% <0.00%> (-1.81%)	⬇️
pyomo/solvers/plugins/solvers/xpress_direct.py	71.75% <0.00%> (-1.07%)	⬇️
pyomo/contrib/pyros/util.py	84.82% <0.00%> (-0.35%)	⬇️
pyomo/contrib/pyros/master_problem_methods.py	88.07% <0.00%> (-0.31%)	⬇️

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[mrmundt]

TIL about mamba. Thanks for teaching me something!

[mrmundt]

This is just a grammar snark, but besides that, it's good.