Skip post setup mambaforge in Continuous Integration

[weiji14]

Description of proposed changes

Reduce time spent by conda-incubator/setup-miniconda on the 'Post Setup Mambaforge' step, especially on Windows which could take ~4-6 minutes.

This feature was added in conda-incubator/setup-miniconda#234, and is available since setup-miniconda=2.2.0 updated in #2203. Default setting is run-post: true which will do 'Removing uncompressed packages to trim down cache folder...'. Setting it to run-post: false shouldn't have any side effects since we're not using any cache.

• Potentially save up to ~6min for Windows Tests, e.g. at https://github.com/GenericMappingTools/pygmt/actions/runs/3611036309/jobs/6085238776#step:27:1, going from ~25min to ~19min.

  

• Potentially save on ~4min for Windows Docs build, e.g. at https://github.com/GenericMappingTools/pygmt/actions/runs/3549154826/jobs/5961196871#step:23:1, going from ~19min to ~15min.

  

Related to #584.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[weiji14]

Example benchmark: windows-latest - Python 3.8 / NumPy 1.21 test took 14m40s at https://github.com/GenericMappingTools/pygmt/actions/runs/3611751950/jobs/6086431051

[seisman]

Looks great!