deploy: add migrate build config
deploy: add migrate build config

This config will be used to run golang-migrate commands
from Cloud Build.

Change-Id: Ia389f772160748dc3f507bdd73fc6233f22afe86
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/367936
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite