Add CI test to check SPDX licenses are up-to-date

[danielmachlab]

Currently, it is a manual process to check that the list of SPDX licenses represented by licenses/spdx/BUILD are up-to-date.

Instead, add a CI test that checks what we have against the authoritative source and fails if we are more than some threshold behind.

See PR review comment: https://github.com/bazelbuild/rules_license/pull/17#discussion_r789099505