cache: respect UserCacheDir convention

[tgross]

The registry cache pollutes the user's home directory with a
~/.nomad directory. Desktop operating systems have platform-specific
conventions for placing cache directories (XDG standard on Unix,
~/Library/Caches on Darwin, %AppData% on Windows, etc.). Go
provides a os.UserCacheDir function that does the right thing for
the platform it's being called on.

---

We can't exactly unit test this change without messing up the test runner's home directory, but here's what it looks like after building locally on macOS:

$ nomad-pack registry list
...
$ ls ~/Library/Caches/nomad/packs/default
csi_openstack_cinder@latest     jenkins@latest                  prometheus_snmp_exporter@latest
drone@latest                    loki@latest                     promtail@latest
faasd@latest                    nginx@latest                    simple_service@latest
fabio@latest                    nomad_autoscaler@latest         tempo@latest
grafana@latest                  nomad_ingress_nginx@latest      tfc_agent@latest
haproxy@latest                  outline@latest                  traefik@latest
hello_world@latest              prometheus@latest               wordpress@latest
influxdb@latest                 prometheus_node_exporter@latest

[DerekStrickland]

I like this suggested change a lot. We struggled with the "right" location, and opted ultimately for a .folder approach based on some other tool's convention, but even that confused some users. This seems very standard to me. I like it.