fix(tvix/eval): fix several string escapings

These were missing an additional level of escaping, silly oversight
caught by an upstream test.

Change-Id: I0312084475e4b88c83945614e9aa5b34c6bc3ec2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6232
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/string.rs

@@ -92,9 +92,11 @@ impl NixString {
 
 fn nix_escape_char(ch: char) -> Option<&'static str> {
     match ch {
-        '\\' => Some("\\"),
-        '"' => Some("\\"),
+        '\\' => Some("\\\\"),
+        '"' => Some("\\\""),
         '\n' => Some("\\n"),
+        '\t' => Some("\\t"),
+        '\r' => Some("\\r"),
         _ => None,
     }
 }