Set `frame_system::LastRuntimeUpgrade` after running `try-runtime` migrations (#2515)

Sets `frame_system::LastRuntimeUpgrade` after running try-runtime
migrations to better emulate real behavior.

This fixes an issue where migrations using the spec version to determine
whether to execute can incorrectly fail idempotency checks.

@s0me0ne-unkn0wn noticed this issue with the session key migration
introduced in https://github.com/paritytech/polkadot-sdk/pull/2265.