pkg_install: provide permission override capability

When working with calling `pkg_install` repeatedly, I found that in cases where permissions are not important, the default is something read-only.

The default copying mechanism in `pkg_install` will blindly assume it can write to the output destination -- if the files already exist and are read-only, installation will fail.

Consider providing something akin to a `--force` flag that will attempt to disregard existing filesystem permission, something like `cp`'s `--force` flag.