release-2.1: teamcity-trigger: plumb TC_BUILD_BRANCH env var into builder

[benesch]

Backport 1/1 commits from #30128.

/cc @cockroachdb/release

---

c0bc52b added a dependency on the TC_BUILD_BRANCH env var to the
teamcity-trigger binary, but the wrapping shell script was not
propagating the variable into the builder's env.

Release note: None

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

This change is 

[andreimatei]

LGTM

…

On Wed, Sep 12, 2018 at 2:25 PM cockroach-teamcity ***@***.***> wrote: This change is [image: Reviewable] <https://reviewable.io/reviews/cockroachdb/cockroach/30133> — You are receiving this because your review was requested. Reply to this email directly, view it on GitHub <#30133 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAXBcTyU1orhjYpxer7AZ7tQAeLREWodks5uaVGxgaJpZM4Wl7NP> .

[andreimatei]

LGTM

…

On Wed, Sep 12, 2018 at 2:25 PM cockroach-teamcity ***@***.***> wrote: This change is [image: Reviewable] <https://reviewable.io/reviews/cockroachdb/cockroach/30133> — You are receiving this because your review was requested. Reply to this email directly, view it on GitHub <#30133 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAXBcTyU1orhjYpxer7AZ7tQAeLREWodks5uaVGxgaJpZM4Wl7NP> .