chore: change Github Action triggers for build/test jobs (#4956)

Github Actions for the build/test jobs (nix.yml, mac.yml, windows.yml) will only run on branches that build packages (develop, release, master), and branches with names starting with "ci/". This is intended as a compromise between disabling CI jobs on personal forks entirely, and having the jobs run as a free-for-all. Note that it will not affect PR jobs at all.

.github/workflows/macos.yml

@@ -1,5 +1,16 @@
 name: macos
-on: [push, pull_request]
+on:
+  pull_request:
+  push:
+    # If the branches list is ever changed, be sure to change it on all
+    # build/test jobs (nix, macos, windows)
+    branches:
+      # Always build the package branches
+      - develop
+      - release
+      - master
+      # Branches that opt-in to running
+      - 'ci/**'
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true