Fix CI: skip cargo-zigbuild install when cached binary works

cargo install fails with exit code 101 when the binary already exists
from cache. Check if cargo-zigbuild works first before installing.

https://claude.ai/code/session_01EcsZDNeWn56wFqryb4Wq7r

.github/workflows/ci.yml

@@ -51,8 +51,7 @@ jobs:
         # ziglang pip package installs as python-zig, symlink to zig for cargo-zigbuild
         sudo ln -sf "$(which python-zig)" /usr/local/bin/zig
         zig version
-        cargo-zigbuild --version 2>/dev/null || cargo install cargo-zigbuild
-        cargo-zigbuild --version
+        cargo-zigbuild --version || cargo install cargo-zigbuild
 
     - run: make check-fmt
     - run: make lint