wrapRustcWith: allow --sysroot to be overridden

It turns out that unlike a normal Unix program, if the --sysroot
option is given more than once, rustc will error rather than using the
last value given.  Therefore, we need to ensure we only add our
default --sysroot argument if one hasn't been given explicitly on the
wrapper's command line.

This fixes cross compilation of rustc.

Closes: https://github.com/NixOS/nixpkgs/issues/271736
Fixes: 8b51cdd3bea1 ("rustc: add a compiler wrapper")

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