CI - Share more steps between workflows/jobs

[bfops]

there are many steps that we copy-paste between workflows. This leads to the possibility of them getting out of sync, and also just makes our workflows longer than necessary.

tl;dr https://chatgpt.com/share/e/691258d4-0290-8011-ab33-4c3cb7fdcd4f

• in SpacetimeDB
• in SpacetimeDBPrivate

[bfops]

Closing in favor of https://github.com/clockworklabs/SpacetimeDBPrivate/issues/2369