Speed up Cirrus CI by using CROSS_TOOLCHAIN

Installing the llvm11 package instead of bootstrapping it from the source
tree reduces the build time by about 20 minutes.

The last freebsd/freebsd build that was tested (r366629) took 1h 21m 22s,
whereas my GitHub fork with this .cirrus.yml took 58m 6s.
We could probably further reduce time by using images that have LLVM
pre-installed: the pkg install step took 4 minutes 30s.

Since the bootstrap toolchain is still tested by Jenkins, this should not
reduce test coverage of the CI testing.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D26747