Fixing several ShellCheck warnings

packaging/source/buildpackage.sh

@@ -2,18 +2,18 @@
 # OpenRA packaging script for versioned source tarball
 
 if [ $# -ne "2" ]; then
-    echo "Usage: `basename $0` tag outputdir"
+    echo "Usage: $(basename "$0") tag outputdir"
     exit 1
 fi
 
 # Set the working dir to the location of this script
-cd $(dirname $0)
+cd "$(dirname "$0")" || exit 1
 
 TAG="$1"
 OUTPUTDIR="$2"
 SRCDIR="$(pwd)/../.."
 
-pushd ${SRCDIR} > /dev/null
+pushd "${SRCDIR}" > /dev/null || exit 1
 make version VERSION="${TAG}"
 git ls-tree HEAD --name-only -r -z | xargs -0 tar cvjf "${OUTPUTDIR}/OpenRA-${TAG}-source.tar.bz2"
-popd > /dev/null
+popd > /dev/null || exit 1