Checkout to release-0.5 branch when e2e v1a5 master is triggered

Checkout to release-0.5 branch when e2e v1a5 master is triggered

E2E v1a5 master jobs are enabled only in CAPM3 repository. In CAPM3
it is release-0.5 branch that holds v1a5 related code base. As such,
for v1a5 master jobs, we need to make sure we default to release-0.5
branch, which will be passed to project-infra scripts, where git
checkout release-0.5 will happen. For v1beta1 master jobs, we continue
checking out to main branch, as configured by default here.

Change-Id: I532a349236f16af6450a7f0325c499c0f35e56c2