build-sys: Adjust for merge of ostree-rs

Fix up the paths for the crates now that the Rust bindings are in
`rust/`.

We can't today include the test suite because it depends on `ostree-rs-ext`
which would make everything circular.

(Building that now requires a separate `cd tests/inst && cargo build`)

tests/inst/Cargo.toml

@@ -4,6 +4,8 @@ version = "0.1.0"
 authors = ["Colin Walters <walters@verbum.org>"]
 edition = "2018"
 
+[workspace]
+
 [[bin]]
 name = "ostree-test"
 path = "src/insttestmain.rs"