Revert "ci: use zig build check-fmt instead of zig fmt --check"

This reverts commit 4d1432299f.

Please don't hard-code unrelated concerns this way. build.zig should not
have awareness of the naming conventions for cmake build directories.

ci/x86_64-linux-debug.sh

@@ -50,8 +50,11 @@ unset CXX
 
 ninja install
 
+# TODO: move this to a build.zig step (check-fmt)
 echo "Looking for non-conforming code formatting..."
-stage3-debug/bin/zig build check-fmt
+stage3-debug/bin/zig fmt --check .. \
+  --exclude ../test/cases/ \
+  --exclude ../build-debug
 
 # simultaneously test building self-hosted without LLVM and with 32-bit arm
 stage3-debug/bin/zig build \