Added a missing rerun-if stanza (#8899)

Author: alex

src/rust/cryptography-cffi/build.rs

@@ -25,6 +25,7 @@ fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
     // FIXME: maybe pyo3-build-config should provide a way to do this?
     let python = env::var("PYO3_PYTHON").unwrap_or_else(|_| "python3".to_string());
+    println!("cargo:rerun-if-env-changed=PYO3_PYTHON");
     println!("cargo:rerun-if-changed=../../_cffi_src/");
     let output = Command::new(&python)
         .env("OUT_DIR", &out_dir)