time: document an alternative way of setting the system's time zone

User's program was mutating time.Local variable and crashing
itself as a consequence. Instead of documenting that time.Local
variable should not be mutated, recommended way of setting the
system's time zone has been documented.

Fixes #34814

Change-Id: I7781189855c3bf2ea979dfa07f86c283eed27091
Reviewed-on: https://go-review.googlesource.com/c/go/+/200457


Reviewed-by: Ian Lance Taylor <iant@golang.org>