cmd/go: fix objdir for run actions for -cover no-test packages
As of CL 495447 we now synthesize coverage data (including coverage
profiles) for packages that have no tests, if they are included in a
"go test -cover" run. The code that set up the "run" actions for such
tests wasn't setting the objdir for the action, which meant that the
coverage profile temp file fragment ("_cover_.out") was being created
in the dir where the test was run, and in addition the same fragment
could be written to by more than one package (which could lead to a
corrupted file). This CL updates the code to properly set the objdir,
and to create the dir when needed.