Fix C headers archive for picolibc

[jprendes]

This PR updates the tar-headers recipe in the Justfile to account for the picolibc changes

[andreiltd]

Could you remind me why do we copy headers from target dir?

[jprendes]

We used to copy the files from different places in the src dir but we already do that when we build the sysroot, I thought it would be easier to copy them from the sysroot directly.
cargo-hyperlight builds the sysroot in the target dir.

[jprendes]

As to why we do this, it's so that we can publish the sysroot in our release, so that people doing C development doesn't need to install Rust.

We publish 2 .tar.gz archives:

• hyperlight-guest-c-api-linux.tar.gz: contains the debug and the release version of hyperlight-guest-capi (and all its dependencies)
• include.tar.gz: contains the libc headers, plus 2 headers generated by cbindgen for hyperlight-guest-capi.