misc/cgo/testshared: run tests only in GOPATH mode
-buildmode=shared installs shared libraries into GOROOT
and expects to reuse them across builds.
Builds in module mode, however, each have their own set of
dependencies (determined by the module's requirements), so in general
cannot share dependencies with a single GOROOT.
Ideally in the long term we would like to eliminate -buildmode=shared
entirely (see #47788), but first we need a replacement for the subset
of use-cases where it still works today.
In the meantime, we should run these tests only in GOPATH mode.
Non-main packages in module mode should not be installed to
GOPATH/pkg, but due to #37015 they were installed there anyway,
and this test heavily relies on installing non-main packages.