[build] delete opam switch directory if it is defunct
[build] delete opam switch directory if it is defunct

Summary:
The GitHub CI is failing again, this time because it seems that we
cannot find our switch listed by opam yet the directory of the switch
exists... Nuke it when that's the case.

Reviewed By: martintrojer

Differential Revision: D37647826

fbshipit-source-id: 66cee8602bba6c0fdd0dc50b01dab3c9900344d8

facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer