Previously, this only applied when using `-fincremental --watch`, but
`--webui` makes the build runner stay alive just like `--watch` does, so
the same logic applies here. Without this, attempting to perform
incremental updates with `--webui` performs full rebuilds. (I did test
that before merging the PR, but at that time I was passing `--watch`
too -- which has since been disallowed -- so I missed that it doesn't
work as expected without that option!)