devtools: make migrate_db.sh respect env vars
devtools: make migrate_db.sh respect env vars

The Postgres contribution guide (/doc/postgres.md) tells the user to
set specific environment variables. When running migrations, the
migrate_db.sh script does not respect the environment variables
referenced. This was causing issues running local migrations.

Changes the script to respect the environment variables if set, and
fall back on postgres defaults if unset.

Change-Id: I5484d825ce2fa503290193aaea403f5cb5ef4cba
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/273927
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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