[ci] Default value for MAKE_ARGS

We don't pass MAKE_ARGS for windows builds, so this should unbreak
them.

.gitlab/ci.sh

@@ -354,9 +354,9 @@ function build_make() {
     fail "BIN_DIST_PREP_TAR_COMP is not set"
   fi
   if [[ -n "${VERBOSE:-}" ]]; then
-    MAKE_ARGS="$MAKE_ARGS V=1"
+    MAKE_ARGS="${MAKE_ARGS:-} V=1"
   else
-    MAKE_ARGS="$MAKE_ARGS V=0"
+    MAKE_ARGS="${MAKE_ARGS:-} V=0"
   fi
 
   echo "include mk/flavours/${BUILD_FLAVOUR}.mk" > mk/build.mk