Proposal: allow installation from a local archive

[mak8kammerer]

There are at least two situations where this feature can be useful:

• Installing a patched Zig compiler;
• Offline installation.

[Anglebase]

I also think it needs to be able to prioritize searching in the local default installation path. I have already installed versions 0.14.1 and 0.15.1 of Zig on my computer, but Zigup still tries to download from the network instead of searching locally first.