change install-sd.sh location for official git repository

With these changes, instead of downloading the install-sd.sh script from https://repo.labriqueinter.net/, the neutrinet_cube_install.sh downloads it from the official git repository.

The first advantage is that it allows us to control which version of the install-sd.sh script we use to flash the SD card (the same way the script downloads a specific version of the hypercube.sh script). Secondly, since it apparently takes quite some time for the updates applied in the git repository to reach https://repo.labriqueinter.net/ (I suppose the maintainers manually update the website), we wouldn't ever suffer from these delays (well, it's hypothetical since there isn't that much commits in the git repository).

The downside is that there is no more additional file integrity check (because the git repo doesn't provide a hash). I hesitated to git clone the full repository to get the install-sd.sh file, but, as the repository is quite big (64Mio), it is kind of a waste for only one little script.