Use thin-lto [1] for "better" release builds.

This performs a rather quick variant of Link Time Optimization [2].
It should add negligible build time but also more optimized binaries.

[1]: https://doc.rust-lang.org/cargo/reference/profiles.html#lto

[2]: https://llvm.org/docs/LinkTimeOptimization.html

.gitlab-ci.yml

@@ -63,6 +63,7 @@ test:cargo:
     # Set some cargo tuning here, because targets overwrite the 'variables'
     - "export CARGO_INCREMENTAL=true"
     - "export CARGO_PROFILE_RELEASE_CODEGEN_UNITS=16"
+    - "export CARGO_PROFILE_RELEASE_LTO=thin"
     - time cargo build --target $TARGET --release
     - 'mv "target/$TARGET/release/conduit" "conduit-$TARGET"'
   artifacts: