cargo: allow building from precompiled sources

[adsharma]

After these changes, we can:

LBUG_PRECOMPILED_RUN_ID=25646256977 cargo build
LBUG_VERSION=0.16.1 cargo build. # will fail, but 0.17.x should work
LBUG_BUILD_FROM_SOURCE=1 cargo build. # what we do currently

Also, the CI for ladybug core repo builds this from a monorepo like setup.