Fix the output path for Rust binaries in the Ninja file to match
Cargo's target directory when CARGO_BUILD_TARGET is set.  This prevents
file-not-found errors for some build steps.

From: Lucio Sauer <watermanpaint@posteo.net>
--- a/build/ninja_gen/src/cargo.rs
+++ b/build/ninja_gen/src/cargo.rs
@@ -61,6 +61,9 @@ impl RustOutput<'_> {
         let mut path: Utf8PathBuf = rust_base.into();
         if let Some(target) = target {
             path = path.join(target);
+        } else {
+            let triple = std::env::var("CARGO_BUILD_TARGET").unwrap_or_default();
+            path = path.join(triple);
         }
         path = path.join(profile_output_dir(build_profile)).join(filename);
         path.to_string()