CI: Set cache-downloads to false to speedup the "Setup Micromamba" step

[seisman]

Currently, we set both cache-downloads and cache-environment options to true in the mamba-org/setup-micromamba action. So, in the "Setup Micromamba" step, we see information like below:

Restoring downloads from `/home/runner/micromamba/pkgs` ...
  Received 209715200 of 614617264 (34.1%), 199.6 MBs/sec
  Received 528482304 of 614617264 (86.0%), 251.7 MBs/sec
  Cache Size: ~586 MB (614617264 B)
  /usr/bin/tar -xf /home/runner/work/_temp/6f85073e-da84-4b9b-a62e-5a406ad9f613/cache.tzst -P -C /home/runner/work/pygmt/pygmt --use-compress-program unzstd
  Received 614617264 of 614617264 (100.0%), 195.2 MBs/sec
  Cache restored successfully
  Restored cache with key `micromamba-downloads--linux-64`
Restoring environment `pygmt` from `/home/runner/micromamba/envs/pygmt` ...
  Received 192937984 of 559585675 (34.5%), 184.0 MBs/sec
  Received 452984832 of 559585675 (81.0%), 216.0 MBs/sec
  Cache Size: ~534 MB (559585675 B)
  /usr/bin/tar -xf /home/runner/work/_temp/5b8debfc-6237-4bc9-ba5b-6f421a5ad7ff/cache.tzst -P -C /home/runner/work/pygmt/pygmt --use-compress-program unzstd
  Received 559585675 of 559585675 (100.0%), 177.8 MBs/sec
  Cache restored successfully
  Restored cache with key `micromamba-environment--linux-64-pygmt-args-62be375-root-dcc80ee`

Apparently, cache-downloads caches/restores the packages in the /home/runner/micromamba/pkgs directory and cache-environment caches/restores the environment from /home/runner/micromamba/envs/pygmt. The cached packages are NOT used in the following steps, thus caching downloads is unnecessary.

This PR changes the cache-downloads option to false in ci_*.yml workflows.

Comparing this branch (https://github.com/GenericMappingTools/pygmt/actions/runs/7384789955/job/20088312682?pr=2946) with the main branch (https://github.com/GenericMappingTools/pygmt/actions/runs/7383547780/job/20084932207), the "Setup Micromamba" step is now faster by a few seconds on Linux and 10-100 seconds faster on Windows.