Use public signing key cache from host

[jvogt]

It would be nice to not have to upload a public origin key to bldr in order to test a hart.

Right now it will fail if you hab setup a new origin on a workstation, build a package, and then try to test it. Looks a bit like this:

$ hab pkg build (which creates results/jvtest-acme_app_1-0.1.0-20190531222846-x86_64-linux.hart)
$ kitchen converge
....
       » Installing jvtest/acme_app_1/0.1.0/20190531222846
       ☛ Verifying jvtest/acme_app_1/0.1.0/20190531222846
       ↓ Downloading jvtest-20190531222734 public origin key
       ✗✗✗
       ✗✗✗ [Err: 0] hab-sup(ER)[components/sup/src/error.rs:459:55]: [404 Not Found]
       ✗✗✗