Remove mac deps; Add RSPM binary support

[schloerke]

Attempting to remove mac deps:

• Related info: macOS brew support? r-system-requirements#81 (comment)
• Related Issue: Add support for mac public RSPM. r-lib/actions#1005
  • Related {terra} page with proof of mac binaries: https://p3m.dev/client/#/repos/cran/packages/overview?search=terra
• Related RSPM impl: https://github.com/r-lib/actions/blob/6b49fefb2846ed3e5e6e58366e7f7dfe01377f02/setup-r/src/installer.ts#L651
• Related GHA sysreqs setup from Gabor: https://github.com/r-hub/actions/tree/main/setup-r-sysreqs
  • Related mac system deps: https://mac.r-project.org/bin/
  • Seemed to not have worked in https://github.com/rstudio/shinycoreci/actions/runs/15077267732/job/42387428677?pr=306

[schloerke]

Seemed to not be working as expected. Currently don't have enough time to debug. Saving for a very rainy day down the road.