dev/build.sh: control build jobs with $ALIRE_BUILD_JOBS (#1651)

This is just a minor quality-of-life improvement for packaging. Note
that this also quotes the argument to echo so that if (somehow)
$ALIRE_OS were set to a malicious value, no harm could be done.

dev/build.sh

@@ -5,7 +5,8 @@ pushd $( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd ) > /de
     . functions.sh
 popd > /dev/null
 
+ALIRE_BUILD_JOBS="${ALIRE_BUILD_JOBS:-0}"
 export ALIRE_OS=$(get_OS)
 
-echo Building with ALIRE_OS=$ALIRE_OS...
-gprbuild -j0 -r -p -P `dirname $0`/../alr_env.gpr "$@"
+echo "Building with ALIRE_OS=$ALIRE_OS..."
+gprbuild "-j$ALIRE_BUILD_JOBS" -r -p -P `dirname $0`/../alr_env.gpr "$@"