deal with github mysteriously running actions with "ubuntu-latest" not resolving to 24.04. check for current ubuntu version and only do bwrap fix it >= 24.04
also, patch flakey flex mirrors, which were not working
There is nothing temporary about our "tmpdir" - its sole purpose is to
contain the final product of the bootstrap process. Thus, removing it
at the end of bootstrap amounts to doing the entire process for nothing.
To remedy this, --tmpdir is renamed --target, keeping the -t short form,
and defaulting to "target" instead of "tmp" to make its purpose clearer.
The --preserve option is removed, as the target is now always preserved.
