Make environs/bootstrap responsible for tools location

environs/bootstrap now finds available tools, augmenting or substituting
for locally buildable/uploadable tools as necessary (auto upload) or as
specified (--upload-tools). Environ.Bootstrap is now provided with a list
of available tools, and is not responsible for syncing tools itself.

--upload-series and --series are obsoleted: we upload a single series,
and then clone that for each supported series. When we store the tools
catalogue in state, we will simply write an entry for each series that
points to the same tools tarball.
Reworked function altogether, hopefully easier to follow now. I also took out the != Ubuntu and made it != version.Current.OS. No reason why Windows shouldn't be able to build for Windows AFAIK.