Fix rm -rf following build symlink into shared cache

rm -rf on a symlink follows it and deletes the target's contents,
which fails when another runner is using the shared cache directory.
Use unlink for symlinks, rm -rf only for real directories.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/scripts/setup-build-cache.sh

@@ -25,7 +25,11 @@ echo "  Cache dir: $_cache_dir"
 
 # Replace any existing build/ (real dir or stale symlink) with a symlink
 # to our runner-specific cache directory.
-if [ -e "build" ] || [ -L "build" ]; then
+# Use unlink for symlinks to avoid rm -rf following the link and deleting
+# the shared cache contents (which another runner may be using).
+if [ -L "build" ]; then
+    unlink "build"
+elif [ -e "build" ]; then
     rm -rf "build"
 fi