misc/cgo/testplugin: unskip test

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>

misc/cgo/testplugin/test.bash

@@ -14,9 +14,6 @@ fi
 goos=$(go env GOOS)
 goarch=$(go env GOARCH)
 
-echo SKIP: golang.org/issue/22571.
-exit 0
-
 function cleanup() {
 	rm -f plugin*.so unnamed*.so iface*.so issue*
 	rm -rf host pkg sub iface