bin/admin: Change init-fs to stop loading a sysroot to init one

bin/admin: Change init-fs to stop loading a sysroot to init one

This is exactly analogous to the `ostree init` case where
we have `OSTREE_BUILTIN_FLAG_NO_REPO` to avoid trying to load
a repo when we're creating one.

Let's avoid the pointless sysroot for `init-fs`; among other
things this will then let us do `ostree_sysroot_load()` inside
the argument parsing, and drop it from every other user.

Closes: #1123
Approved by: jlebon