refactor(.github/workflows): reorganize many builds (#332)

1. reduce the number of periodic builds

2. just build as part of the release process in most cases

3. shorttests duplicates coverage

Preliminary changes as part of https://github.com/ooni/probe/issues/1466

.github/workflows/ios.yml

@@ -1,10 +1,9 @@
-# ios verifies we can still build for iOS
+# ios checks whether we can build for iOS
 name: ios
 on:
   push:
     branches:
-      - 'release/**'
-      - 'master'
+      - "release/**"
 jobs:
   test:
     runs-on: macos-10.15