go/build: in TestImportDirTarget, only expect an install target when cgo is enabled

go/build: in TestImportDirTarget, only expect an install target when cgo is enabled

As of CL 448803, packages in GOROOT only have install targets when
they have cgo source files. When cgo is not enabled, that condition
is necessarily false, and no install target will exist.

For #47257.

Change-Id: I653a9c5f89d18a5841810f3de8d490bd7cb7e922
Reviewed-on: https://go-review.googlesource.com/c/go/+/449375
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>