feat: build ooniprobe for armv6 (#904)

Part of https://github.com/ooni/probe/issues/1753.

While there, introduce a rule by which, if the branch is named `fullbuild` we run all possible builds. It helps to test all the builds without creating a release branch. Because release branches are protected, they cannot be deleted easily. On the contrary, the `fullbuild` branch can easily be disposed of.

.github/workflows/android.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - "release/**"
+      - "fullbuild"
     tags:
       - "v*"