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/jafar.yml

@@ -2,12 +2,10 @@ name: jafar
 on:
   push:
     branches:
-      - 'release/**'
-  schedule:
-    - cron: "0 5 * * 3"
+      - "release/**"
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/setup-go@v1
         with: