The vet action assumes that a.Deps[0] is the compilation action for
which vet information should be generated. However, when using
-linkshared, the action graph is built with a ModeBuggyInstall action
to install the shared library built from the compilation action.
Adjust the set up of the vet action accordingly. Also don't clean up
the working directory after completing the buggy install.

Updates #26400

Change-Id: Ia51f9f6b8cde5614a6f2e41b6207478951547770
Reviewed-on: https://go-review.googlesource.com/124275
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>