The test TestGoGetHTTPS404 downloads a package that does not build on
every OS, so change it to only run where the package builds. It's not
great for the test to depend on an external package, but this is an
improvement on the current situation.

Fixes #15644.

Change-Id: I1679cee5ab1e61a5b26f4ad39dc8a397fbc0da69
Reviewed-on: https://go-review.googlesource.com/23920
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>