switch Actions workflows to use micromamba

[prjemian]

When running the Actions workflows, most of the time is spent in the conda solver. The micromamba solver is remarkably faster. Use it.

[prjemian]

See https://github.com/marketplace/actions/provision-with-micromamba

[prjemian]

Since this resolves #233, apply micromamba to other workflows.

[prjemian]

The micromamba provisions a custom environment name, so no need for the .github/edit_test_environment.py code now.

[prjemian]

Note that time to complete unit tests workflow (successfully) has decreased to ~3 minutes. Before these changes, the time was ~5 minutes.