chore(tvix/cli): silence useless warning

Change-Id: I18f567d5dcbbeb934428bf9111eb5fae5fef0342
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8261
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/cli/src/known_paths.rs

@@ -164,6 +164,7 @@ impl KnownPaths {
     }
 
     pub fn add_replacement_string<D: ToString>(&mut self, drv: D, replacement_str: &str) {
+        #[allow(unused_variables)] // assertions on this only compiled in debug builds
         let old = self
             .replacements
             .insert(drv.to_string(), replacement_str.to_owned());