This ensures that `zig fetch --save` will work properly.

The older lookup used a inline for instead of a simple switch

`OpenDirOptions` has been deprecated.

This switches from to `goto-bus-stop/setup-zig` to `mlugg/setup-zig`

This removes the arena allocator and prepares the code for the next commit that will also remove some allocations

…for testing purposes

This also allows us to `std.posix.getenv` instead of `std.process.getEnvVarOwned` on linux.

Thanks to the "System" abstraction, all possible environment variables and `user-dirs.dirs` can be tested.

gyro has been archived