brillo_config: Don't cache the timestamp.

We want the timestamp to change at every build as we may be using the
resulting image to generate an update and we need different images to
have different versions.

Bug: 25358721
Change-Id: I9ae0ae0ba34ea0f508fc3a5f051368e4bce57b61
Test: Build twice in a row, the timestamp is updated.
1 file changed