Instead of allocating a volume for each individual Buildroot-based
project, I think it will be easier to reuse the same one for all of
them. It's not like we can really run more than one job at a time,
anyway.
Buildroot jobs really benefit from having a persistent workspace volume
instead of an ephemeral one. This way, only the packages, etc. that
have changed since the last build need to be built, instead of the whole
toolchain and operating system.
