poll: fix printf: non-constant format string in call (govet)

    poll/poll.go:154:18: printf: non-constant format string in call to gotest.tools/v3/poll.Continue (govet)
        return Continue(buf.String())
                        ^

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

poll/poll.go

@@ -151,7 +151,7 @@ func Compare(compare cmp.Comparison) Result {
 	if assert.RunComparison(buf, assert.ArgsAtZeroIndex, compare) {
 		return Success()
 	}
-	return Continue(buf.String())
+	return Continue("%v", buf.String())
 }
 
 type logBuffer struct {