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CL 99175 added TestVetWithOnlyCgoFiles. However, this
test is failing on platforms where cgo is disabled,
because no file can be built.

This change fixes TestVetWithOnlyCgoFiles by skipping
this test when cgo is disabled.

Fixes #24304.

Change-Id: Ibb38fcd3e0ed1a791782145d3f2866f12117c6fe
Reviewed-on: https://go-review.googlesource.com/99275

Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>