ci: filter build-examples to directories with build.zig.zon

The dynamic example directory discovery added in bb3b3ba included
all subdirectories under example/, but some (wasm-key-encode,
wasm-sgr) are pure HTML examples with no build.zig.zon. Running
zig build in those directories falls back to the root build.zig
and attempts a full GTK binary build, which fails on CI.

Filter the listing to only include directories that contain a
build.zig.zon file so non-Zig examples are excluded from the
build matrix.

.github/workflows/test.yml

@@ -179,7 +179,7 @@ jobs:
       - id: list
         name: List example directories
         run: |
-          dirs=$(ls -d example/*/ | xargs -n1 basename | jq -R -s -c 'split("\n") | map(select(. != ""))')
+          dirs=$(ls example/*/build.zig.zon 2>/dev/null | xargs -n1 dirname | xargs -n1 basename | jq -R -s -c 'split("\n") | map(select(. != ""))')
           echo "$dirs" | jq .
           echo "dirs=$dirs" >> "$GITHUB_OUTPUT"