rapids-rattler-channel-string returns an array of string, not a string

This is to allow expanding these arguments to `rattler-build` in a
controlled way, (`rattler-build build ... "${RATTLER_CHANNELS[@]}"`)

tools/rapids-rattler-channel-string

@@ -22,5 +22,5 @@ _add_c_prefix() {
 
 rapids-logger "Using channels: ${channels[*]}"
 
-RATTLER_CHANNELS=$(_add_c_prefix)
+RATTLER_CHANNELS=($(_add_c_prefix))
 export RATTLER_CHANNELS