macos - perform a builddir cleanup at the start of the run

If the connection is lost in a non-clean fashion then the build dir won't be cleaned
see: https://buildbot.mariadb.org/#/builders/590/builds/14911

the next run will fail and perform the cleanup as a last step: https://buildbot.mariadb.org/#/builders/590/builds/14914

the third run will run as expected (starting with an empty build dir):
https://buildbot.mariadb.org/#/builders/590/builds/14916

TODO:

• perform the cleanup at the start of the run also