[electrum] consistently download the locale files from github

Summary: The linux AppImage and Windows build scripts use this way of downloading the locales, from a github repository with a specific checkpoint. Do the same thing for the source release and MacOS release, instead of attempting to fetch the latest translations from Electron Cash's crowdin repo (which have likely diverged a bit since the fork). This fixes a build failure after a recent change in crowdin's API.

Test Plan:
contrib/build-linux/srcdist_docker/build.sh

On a MacOS VM:
contrib/osx/make_osx

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16718