Add target to Env struct

[jcmoyer]

Fixes an issue where zls couldn't parse the output from zig env causing it to repeatedly print

debug: (store ): Cannot resolve std library import, path is null.

and fail to autocomplete.

See: ziglang/zig#11741

[SuperAuguste]

Website CI is broken; I'll merge this when it's fixed :)

[leecannon]

This will need to be target: ?[]const u8 = null, to work with older zig versions, including CI atm.

[nektro]

set ignore_unknown_fields = true in our std.json.ParseOptions instead

[jcmoyer]

I'm not sure which behavior is more preferrable (add target field or use ignore_unknown_fields). I only became aware of the zig env change explicitly because ignore_unknown_fields wasn't being used.