nix: limit jobs used by parallel to update Gradle

Otherwise on some devices with with good connecitons rate limiting might
cause failures to fetch POMs or JARs and in result failing the whole update.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

nix/deps/gradle/generate.sh

@@ -52,7 +52,8 @@ function gen_deps_json() {
 
     # Format URLs into a Nix consumable file.
     URLS=$(cat ${DEPS_URLS})
-    parallel --will-cite --keep-order \
+    # Avoid rate limiting by using 4 of the available threads.
+    parallel --will-cite --keep-order --jobs 4 \
         "${CUR_DIR}/url2json.sh" \
         ::: ${URLS} \
         >> ${DEPS_JSON}