[recipes] Stop pretending that buildbot_spec is a step, simplify

Since we no longer have to call out to obtain the buildbot_spec, merge
the two functions in vars API.  As a side effect, this applies
default_env to the sync steps as well, which shouldn't have an
appreciable effect on bot behavior.

BUG=skia:5578
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2209343002

Review-Url: https://codereview.chromium.org/2209343002
25 files changed