Function compute_key 

pub fn compute_key(
    run: &str,
    extra_keys: &[String],
    input_files: &[(PathBuf, Vec<u8>)],
) -> String

Compute a deterministic SHA-256 cache key from:

• the command’s run script
• any extra key strings declared in the command cache config
• the path and content of every input file (in sorted path order)