fix syntax issue

Author: nitins17

ci/build_artifacts.sh

@@ -54,7 +54,7 @@ if [[ $os =~ "msys_nt" ]] && [[ $arch == "x86_64" ]]; then
 fi
 
 # Use "rbe_" config for Linux x86/Windows and "ci_" for other platforms
-bazelrc_config="$os_$arch"
+bazelrc_config="${os}_${arch}"
 if ( [[ "$os" == "linux" ]] && [[ "$arch" == "x86_64" ]] ) || [[ "$os" == "windows" ]]; then
   bazelrc_config="rbe_$bazelrc_config"
 else