fix(cust_raw): qualify str::from_utf8 to avoid nightly-only inherent

[brandonros]

error[E0658]: use of unstable library feature `inherent_str_constructors`
   --> /home/user/.cargo/git/checkouts/rust-cuda-fad079f24bbca397/103a8d5/crates/cust_raw/build/callbacks.rs:173:28
    |
173 |             let expanded = str::from_utf8(&expanded).unwrap();
    |                            ^^^^^^^^^^^^^^
    |
    = note: see issue #131114 <https://github.com/rust-lang/rust/issues/131114> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `cust_raw` (build script) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

gets rid of this