Relative cache dir and build prefix

[perillo]

The purpose of this changeset is to allow the user to override the default zig-cache and zig-out directories.

The next step is to add the ZIG_INSTALL_DIR environment variable, so that both zig-cache and zig-out can be overridden using environment variables. I'm not sure if this is the correct way, however.

Some issues:

• I used var zig_cache = &@as([]const u8, "zig_cache") to store a string literal in a mutable variable, but I'm not sure if there is a better way.

• zig fmt have zig-cache and zig-out hard coded, so that these directories are ignored. Using a custom cache directory, zig fmt may modify the cache entries.

• Some user may depending on the current behavior for relative --cache-dir and --prefix. However in this case it possible to do (like in the ci scripts): export ZIG_LOCAL_CACHE_DIR="$(pwd)/zig-local-cache"

  By the way, shellharden suggests to use $PWD instead of $(pwd).

[perillo]

I would like to add some tests. Should I add them to test/cli.zig?

[andrewrk]

Closing abandoned PR (CI checks failing).