cmd/go: add a control case to the mod_vendor_trimpath test

In reviewing CL 215940, it took me a while to find the control
condition for the test, which was located in build_cache_trimpath.txt.

We could consolidate the two tests, but since they check for
regressions of separate issues (with separate root-causes), I think it
makes sense to keep them separate.

However, I would like the control condition to be present in the same
source file, so that we'll be more likely to update both cases if the
behavior of one of them is changed.

Updates #36566

Change-Id: Ic588f1dfb7977dd78d1d5ef61b9841e22bad82e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/216018


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>