cargo: Default to thin lto for the release profile

Normal "fat" lto is quite resource heavy and requires
a lot of ram at compile time. Thinlto is a
much lighter alternative that produces comparable results
performance but it scales much better. [1] [2] [3]

All browser engines also compile with thinlto exclusively,
as it's almost as fast and saves considerable amount of
link time in the build.

[1]: https://doc.rust-lang.org/cargo/reference/profiles.html#lto
[2]: https://clang.llvm.org/docs/ThinLTO.html
[3]: https://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html

In a test build on a windows machine, this brings down
the from scratch compile time from 7m30s to roughly 5m.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1922>

Cargo.toml

@@ -117,7 +117,7 @@ default-members = [
 ]
 
 [profile.release]
-lto = true
+lto = "thin"
 opt-level = 3
 debug = true
 panic = 'unwind'