The test TestNewReleaseRebuildsStalePackagesInGOPATH is not run in
short mode, so people tend to not notice when it fails. It was failing
due to the build cache. Make it pass again by 1) changing it to modify
the package in a way visible to the compiler, so that the change is
not hidden by caching; 2) accepting "not installed but available in
build cache" as always being a valid reason for a stale package, as go
list does not try to figure out an underlying reason for why a package
is stale when it finds it in the build cache but not installed.

Updates #24436

Change-Id: Iaeaa298f153451ec913a653dd4e6da79a33055bb
Reviewed-on: https://go-review.googlesource.com/123815Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan C. Mills <bcmills@google.com>