Fixed building animations staying paused when disabled due to low power during sell causing glitches

Both #6076 and #6275 are duplicates. Probably introduced by myself in https://github.com/OpenRA/OpenRA/pull/5138.
`IsDisabled` is an extension method, which are static methods paired with some special compiler logic that allows `FooExts.DoBaz(this Foo foo, Bar bar)` to be called via `foo.DoBaz(bar)`.  This doesn't cache anything, and so the body of `IsDisabled` does a `TraitsImplementing` each time it is called.