Do not allow concurrent build on post-submit
Do not allow concurrent build on post-submit

With automatic push, we will have an increase of post-submit, so avoid
overloading the post submit queue by disallowing concurrent build.

Change-Id: Iaf71ba4a1222de0e82803443f74ce451f73594c4

bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration