Add needed pip packages to src/environment.yml, minimize conda environments

[mkoeppe]

📚 Description

• Adds a pip section to src/environment.yml for the missing standard package sagenb_export (this was left out in bootstrap-conda: Refactor, generate versioned environment files #36405)
• This pip section is also good for adding the new pip package in Update to new conway-polynomials python package #36765
• Resolves Minimize conda environment #34626
• Update CI conda to use pkgs/sage-conf_conda (= what is documented in the installation guide)
• Update pkgs/sage-conf_conda to use --enable-system-site-packages (this was left out in Simplify experimental all-conda installation instructions via pkgs/sage-conf_conda #36367 (review)) and to avoid --with-system-SPKG=force for packages that may be omitted by the environment minimizer.
• Fixes some failures with src/environment-optional.yml

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

• Depends on sage --package list: Sort output, add switches --{in,ex}clude-dependencies #36393
• Depends on bootstrap-conda: Refactor, generate versioned environment files #36405
• Depends on Fix conda install instructions after setuptools update #36411
• Depends on build/pkgs/setuptools_scm: Update to 8.0.4, add fixes for version 8 #36400
• Depends on Fix mamba install in conda ci workflow #36468
• Depends on Remove conda patchelf distro #36499 (split out from here)
• Depends on networkx, scipy, ipywidgets: Update version ranges in conda.txt #36513 (split out from here)
• Depends on build/pkgs/sagenb_export: Fix install-requires.txt #36514 (split out from here)
• Depends on ipython: Fix conda version range #36560 (split out from here)

[mkoeppe]

Our upper bound on setuptools conflicted with cvxpy, which for unknown reasons has setuptools in install-requires (see cvxpy/cvxpy#2118)

[mkoeppe]

Still lots of conflicts, involving narrowly restricted version of boost-cpp etc.

[mkoeppe]

@tobiasdiez do you think we can have the "s: run conda ci" label also run the workflow on PR syncs?

[github-actions]

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