atc: structure: fix up data race for build tracker
atc: structure: fix up data race for build tracker

rather than doing a full DeepEqual on the entire fake build, just
compare IDs

this failed with --race because the goroutines call .ID(), which with a
fake object causes mutation for all the internal bookkeeping around call
count / call args, resulting in a data race.

Signed-off-by: Alex Suraci <925ba519bbf1a06224928b692ef88b93aee946af@gmail.com>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse