testground is a cyclic dependency

[galargh]

We depend on testground here https://github.com/testground/sync-service/blob/master/go.mod#L8 while testground depends on sync-service through sdk-go https://github.com/testground/testground/blob/master/go.mod#L33 and https://github.com/testground/sdk-go/blob/master/go.mod#L14

This prevented me from updating plan-templates in https://github.com/testground/testground/blob/master/go.mod#L32

cc @laurentsenta