fix: set extensible target=$arch in site config

containers/jug/dev.Dockerfile

@@ -49,7 +49,7 @@ RUN declare -A arch=(                                                   \
       ["linux/arm64"]="aarch64"                                         \
     )                                                                   \
  && arch=${arch[${TARGETPLATFORM}]}                                     \
- && spack config --scope site add "packages:all:require:arch=${arch}"   \
+ && spack config --scope site add "packages:all:require:[arch=${arch}]" \
  && spack config blame packages                                         \
  && spack config --scope site add "config:suppress_gpg_warnings:true"   \
  && spack config --scope site add "config:build_jobs:${jobs}"           \