fix(tvix/eval): off-by-one in replaceStrings

replaceStrings would previously fail to replace the last character
in a string.

Change-Id: I43a7c960945350b2e7a5b731b7fdb617723eb38f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9151
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -723,7 +723,7 @@ mod pure_builtins {
                 let from = elem.0.to_str()?;
                 let to = elem.1.to_str()?;
 
-                if i + from.len() >= string.len() {
+                if i + from.len() > string.len() {
                     continue;
                 }