devtools: Bump timeout for v23-proxy-integration-test to 30 min.
devtools: Bump timeout for v23-proxy-integration-test to 30 min.

Attempt to fix https://github.com/vanadium/build/issues/21, which looks
like it is caused by a timeout.

Change-Id: I0be688d26ad858b3ffe4cf744dc00c272a8afb5e

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools