Use default branch of `psvm` when synchronizing templates (#4240)

We cannot lock to a specific version of `psvm`, because we will need to
keep it up-to-date - each release currently requires a change in `psvm`
such as [this one](https://github.com/paritytech/psvm/pull/2/files).

There is no `stable` branch in `psvm` repo or anything so using the
default branch.