zap: cache_clean: get apk.static if missing (MR 2481)

After extending CI to always run "pmbootstrap zap -a" it revealed
another bug: we did not ensure at this point that apk.static is
available. Rung apk_static.init() to ensure it gets downloaded and
extracted at this point if it is missing.

pmb/helpers/apk.py

@@ -273,6 +273,7 @@ def cache_clean(arch: Arch) -> None:
     command += ["cache", "clean"]
     _command = _prepare_cmd(command, None)
 
+    pmb.helpers.apk_static.init()
     pmb.helpers.run.root(_command)