Change default PREFIX to /opt/local

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

rocky9/build.sh

@@ -3,7 +3,7 @@
 set -eu
 
 IMAGE_NAME="${IMAGE_NAME:-rocky9-source-stack}"
-PREFIX="${PREFIX:-/usr/local}"
+PREFIX="${PREFIX:-/opt/local}"
 SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
 SOURCES_DIR="${SOURCES_DIR:-$SCRIPT_DIR/../sources}"
 FETCH_SCRIPT="$SCRIPT_DIR/../fetch-sources.sh"