cmd/go: in script tests, avoid checking non-main packages for staleness

cmd/go: in script tests, avoid checking non-main packages for staleness

Non-main packages in module mode should not be installed to
GOPATH/pkg, but due to #37015 they were installed there anyway.
Lacking a proper install location, 'go install' becomes a no-op
for non-main packages in module mode.

This change switches the 'go install' commands in the test_fuzz_cache
and build_overlay tests to instead use 'go build', using the '-x' flag
to check for compile commands instead of querying 'go list' about
staleness.

For #37015.

Change-Id: I56d80cf2a43efb6163c62082c86cd3e4f0ff73c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/417095
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>