CI: fix tests.yml yaml parse error in yarn-berry setup heredoc

The previous commit's inline heredoc put @echo off at column 1,
which YAML tries to parse as the start of a token @ is a reserved
indicator). GitHub Actions rejected the whole workflow as
malformed, making every test job skip.

Replace the heredoc with a single printf call that keeps the
body inside the YAML block-scalar's indentation — functionally
equivalent but parseable.

Author: claude

.github/workflows/tests.yml

@@ -179,11 +179,10 @@ jobs:
         echo "$YARN_BERRY_ALIAS_DIR" >> "$GITHUB_PATH"
         npm install --prefix "$YARN_BERRY_PREFIX" @yarnpkg/cli-dist@4.13.0
         # Emit a one-line ``yarn-berry.cmd`` that forwards to npm-installed
-        # ``yarn.cmd``. Use ``%*`` to pass through all args.
-        cat > "$YARN_BERRY_ALIAS_DIR/yarn-berry.cmd" <<EOF
-@echo off
-"$YARN_BERRY_PREFIX\\node_modules\\.bin\\yarn.cmd" %*
-EOF
+        # ``yarn.cmd``. Use ``%*`` to pass through all args. Built with
+        # printf so the heredoc body doesn't collide with YAML block-scalar
+        # indentation.
+        printf '@echo off\n"%s\\node_modules\\.bin\\yarn.cmd" %%*\n' "$YARN_BERRY_PREFIX" > "$YARN_BERRY_ALIAS_DIR/yarn-berry.cmd"
         export PATH="$YARN_BERRY_ALIAS_DIR:$PATH"
         command -v yarn-berry
         yarn-berry --version | grep -q '^4\.'