Build directories in the cache are ignored

[jeremyfowers]

filesystem.get_available_builds() looks for a .turnkey_build file marker to assess whether a directory is a build directory. There are a variety of situations where .turnkey_build is excluded, which then causes issues. For example, turnkey cache report doesn't care about .turnkey_build but turnkey cache delete --all does check for it.

The call to create_build_directory() was placed where it was, in analyze/script.py because we were under code freeze for the build tool at the time. As you can see in the comment, this was not the right long-term home for it.

The placement of that call lead to a bug: if the process doing the build is killed for any reason (e.g., a timeout), then that finally block will never execute and the directory will not count as a build because it doesn't have a .turnkey_build marker in it. In turn, this impacts functions like filesystem.get_available_builds() that check for that marker (ie, such a build shouldn't show up in turnkey cache list).

Solution: move the LoC to the right long term home: the moment the build is actually being created: build/ignition/py::_begin_fresh_build()