xtask: Fix rand 0.10 API compatibility

In rand 0.10, the `random_range` method moved from the `Rng` trait
to the new `RngExt` trait. Update the import accordingly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: henrywang

Cargo.lock

@@ -1,7 +1,7 @@
 use anyhow::{Context, Result};
 use camino::{Utf8Path, Utf8PathBuf};
 use fn_error_context::context;
-use rand::Rng;
+use rand::RngExt;
 use xshell::{Shell, cmd};
 
 // Generation markers for integration.fmf