stage2: Fix arg processing for zig run
stage2: Fix arg processing for zig run

* Stop parsing arguments after `--`
* Calculate the correct index for the first argument after `--`

zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig