Gitian: refresh submodules before compiling

[mj-xmr]

Prevents symptoms such as:

Submodule 'external/miniupnp' is not up-to-date.

during performing "Reproducible Builds".

The problem lies in gitian-builder, where the submodule synchronisation is not done thoroughly:

system!("cd inputs/#{dir} && git submodule update --init --recursive --force")

Please compare with my version:

    system!("cd inputs/#{dir} && git submodule update --init --recursive --force")
    system!("cd inputs/#{dir} && git submodule update --remote")
    system!("cd inputs/#{dir} && git submodule sync")
    system!("cd inputs/#{dir} && git submodule update")

[EDIT]: A much less obtrusive alternative can be found here: #8296

[mj-xmr]

Closing, since the equivalent #8296 is much simpler.