Assets that skip concatenation should still get an `mtime`

If something isn't concatenated, it bypasses the addition of an `mtime` as well, which can lead to stale versions of those assets remaining in cache: https://github.com/Automattic/nginx-http-concat/blob/2239f115f3da6d09c8660db65163a08e55d0348c/cssconcat.php#L107.

Start in cssconcat.php around line 107, where non-concatenated assets bypass the mtime addition. Trace that path and verify that an mtime is still included for assets that skip concatenation, so cached assets can be invalidated when they change.

Written by the indexing model from the issue text.