fix: use cargo linker config key instead of rustflags for path resolution

The linker path specified via rustflags (-C linker=...) is passed to
rustc which doesn't resolve it relative to the config file. Using
Cargo's native `linker` key resolves relative paths correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: branchseer

.cargo/config.toml

@@ -7,7 +7,7 @@ bindeps = true
 # Linker wrappers for musl targets. On Linux hosts these use the system cc directly;
 # on non-Linux hosts (macOS, Windows) they cross-compile via cargo-zigbuild.
 [target.x86_64-unknown-linux-musl]
-rustflags = ["-C", "linker=.cargo/zigcc-x86_64-unknown-linux-musl"]
+linker = ".cargo/zigcc-x86_64-unknown-linux-musl"
 
 [target.aarch64-unknown-linux-musl]
-rustflags = ["-C", "linker=.cargo/zigcc-aarch64-unknown-linux-musl"]
+linker = ".cargo/zigcc-aarch64-unknown-linux-musl"