buildroot: remove use of cached tarballs
buildroot: remove use of cached tarballs

Cached tarballs were in a directory only writable by lbedford,
which made builds break for other users.

R=lvd
I=4589048

denidoank/avalonsailing,SteffenGrundmann/avalonsailing,SteffenGrundmann/avalonsailing,denidoank/avalonsailing,SteffenGrundmann/avalonsailing,SteffenGrundmann/avalonsailing,denidoank/avalonsailing,denidoank/avalonsailing,SteffenGrundmann/avalonsailing,SteffenGrundmann/avalonsailing,denidoank/avalonsailing,denidoank/avalonsailing