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/e2eminiooni.yml

@@ -1,15 +1,12 @@
-# Run end-to-end testing using miniooni
+# e2eminiooni runs end-to-end tests using miniooni
 name: "e2eminiooni"
 on:
-  pull_request:
   push:
     branches:
-      - master
-  schedule:
-    - cron: "25 */8 * * *"
+      - "master"
 jobs:
   test:
-    runs-on: "ubuntu-latest"
+    runs-on: "ubuntu-20.04"
     steps:
       - uses: actions/setup-go@v1
         with: