fix(corp/tvixbolt): set output to eval value.

This CL fixes the bug where output of a nix
evaluation is not set.

Change-Id: I8ae2759a7ec26e1de2e57dd43302129347a8c302
Signed-off-by: Aaqa Ishtyaq <aaqaishtyaq@gmail.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7896
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

corp/tvixbolt/src/main.rs

@@ -296,6 +296,11 @@ fn eval(model: &Model) -> Output {
         }
     }
 
+    out.output = match result.value {
+        Some(val) => val.to_string(),
+        None => "".to_string(),
+    };
+
     for warning in result.warnings {
         writeln!(
             &mut out.warnings,