cirrus: Add missing depends_sources_cache to Android task

This cache entry is required for completeness. The file
src/Makefile.qt.include needs it in this line:
 QT_BASE_PATH = $(shell find ../depends/sources/ -maxdepth 1 -type f -regex ".*qtbase.*\.tar.xz")

This cache entry is tied to the depends_built_cache cache entry. Either
both are present and cached, or neither of them is. Otherwise, the build
will fail.

.cirrus.yml

@@ -183,6 +183,8 @@ task:
 
 task:
   name: 'ARM64 Android APK [focal]'
+  depends_sources_cache:
+    folder: "/tmp/cirrus-ci-build/depends/sources"
   << : *GLOBAL_TASK_TEMPLATE
   container:
     image: ubuntu:focal