zig env: improve envvar handling and add command line args support

[silversquirl]

I noticed zig env doesn't properly handle the ZIG_LIB_DIR envvar, so this fixes that. I also added --zig-lib-dir and --global-cache-dir args, because eg. a wrapper script for zig might pass those in.

[mlugg]

ZIG_LIB_DIR env var handling was dealt with by #24012.

I don't understand the rationale for adding the --zig-lib-dir and --global-cache-dir args. Even if your zig is a wrapper script, it cannot just blindly forward options like --zig-lib-dir to arbitrary subcommands; it's nonsensical for a bunch of them, such as all of the "drop-in replacement" tooling (zig ar, zig cc, zig objcopy, etc), and also things like zig fmt. If you want to override your lib directory or cache directory globally, the way to do that isn't by injecting CLI arguments: it's using the environment variables!

Do you have a specific use case which would be solved by introducing these arguments?

[silversquirl]

happy with the resolution in #24012! hadn't seen that pr ^-^