Fix an old unnoticed bug in Bisectable

This code seems to be bitrotted, but the latest changes were in 2018. It
looks like nobody noticed because we simply had no missing builds.

lib/Whateverable/Bisection.pm6

@@ -46,7 +46,8 @@ sub run-bisect(&runner  = &standard-runner,  #← Something to run on every revi
 
         if $skip-missing-builds and not build-exists $current-commit {
             take ‘»»»»» Build does not exist, skip this commit’;
-            return Skip # skip non-existent builds
+            take get-output(cwd => $repo-cwd, <git bisect>, Skip.lc)<output>;
+            next
         }
 
         my $run-result     = &runner(              :$current-commit, |%custom);