run.bat: do not unset GOROOT_FINAL before running tests

This removes the same logic from run.bat that was removed from
cmd/dist in CL 236819.

The duplicated logic was removed from run.bash and run.rc in CL 6531,
but that part of run.bat was apparently missed (and not noticed
because its effect was redundant).

Also fix a path-separator bug in cmd/addr2line.TestAddr2Line that was
exposed as a result.

Fixes #39478
Updates #39385

Change-Id: I00054966cf92ef92a03681bf23de7f45f46fbb5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/237359


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>