rust: Install target.json for target rustc

As part of the merging builds, installation of target.json was lost
causing `rustc` to fail on startup with:

  error: Error loading target specification: Could not find specification for target "aarch64-poky-linux-gnu". Run `rustc --print target-list` for a list of built-in targets

We know the full target specification, so just install it directly
rather than using the glob approach from previously.

Fixes: b9b0cd99 ("rust: Enable nativesdk and target builds + replace rust-tools-cross-canadian")
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>