Fixed by CL 76025 yesterday, without realizing it:
the testshared and testplugin builds of separate iface_i
packages were colliding incorrectly in the cache.
Including the build directory fixes that.

Fixes #22571.

Change-Id: Id8193781c67c3150823dc1f48eae781dfe3702fb
Reviewed-on: https://go-review.googlesource.com/76371
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>