Use a conda lock file to improve CI reliability

[arthurp]

Conda does not natively support lock files, but they are easy to implement as a separate tool: https://github.com/conda-incubator/conda-lock. Using lock files will have two main benefits:

• By locking the versions we avoid upstream releases causing CI failures.
• Conda dependency resolution will be faster since many packages are pinned to an exact version. (Conda dependency resolution uses a SAT solver, so it can be very slow.)

The lock file is easy to use for development builds (locally or in CI), however there is no support for using the lock file to build conda packages. This is reasonable because doing so would limit the compatibility of the resulting conda packages since it was not built with the most recent versions of other packages.

Semi-automatic updates to the lock file would be relatively easy. (A script could update the lock file and create PR for human review.)

[arthurp]

Conda dependency resolution will be faster since many packages are pinned to an exact version. (Conda dependency resolution uses a SAT solver, so it can be very slow.)

This is less critical now because CI uses mamba. But a lock file would further help I think.

Semi-automatic updates to the lock file would be relatively easy. (A script could update the lock file and create PR for human review.)

This could reuse the scripts/version code that creates PRs for new version numbers.