With vendored source, use cargo build --offline

This way we're sure it will build in e.g. Koji.  Right now
it's annoying to test that locally; one needs to explicitly
create a no-network container to do so strictly.  But
cargo has a convenient `--offline` flag, and nothing else
in our build stack should touch the network.

Makefile-rpm-ostree.am

@@ -100,6 +100,9 @@ privdata_DATA = src/app/rpm-ostree-0-integration.conf
 
 # Propagate automake verbose mode
 cargo_build = $(cargo) build $(if $(subst 0,,$(V)),--verbose,)
+if BUILDOPT_PREBUILT_BINDINGS
+cargo_build += "--offline"
+endif
 
 if BUILDOPT_ENABLE_SQLITE_RPMDB_DEFAULT
 cargo_build += --features sqlite-rpmdb-default