sysutils/loki: Pin to go1.19

When built with go1.20, loki fails at runtime:

panic: Something in this program imports
go4.org/unsafe/assume-no-moving-gc to declare that it assumes a
non-moving garbage collector, but your version of
go4.org/unsafe/assume-no-moving-gc hasn't been updated to assert that
it's safe against the go1.20 runtime. If you want to risk it, run with
environment variable ASSUME_NO_MOVING_GC_UNSAFE_RISK_IT_WITH=go1.20 set.
Notably, if go1.20 adds a moving garbage collector, this program is
unsafe to use.

Pin this port to go1.19 until this is resolved upstream.

Reported by: cation (via #freebsd-ports)
Approved by: portmgr (blanket, just fix it)