Wrong compilation target when building guest crates that use cc-rs

When we build a guest crate that has some C dependencies built with cc-rs, the intention is to use the x86_64-unknown-linux-none, as explained in the comments in hyperlight-guest-bin's build.rs.
However, when invoking clang, cc-rs will pass the --target=x86_64-unknown-none target.
As the clang wrapper flags are PREPENDED in the in the actual clang invokation, the cc-rs --target overrides the wraper's target.