Custom-build: Document breaking change on built comps.
As mentioned by
e8c9d19, Cabal 3.12 introduced a breaking change where
an unnecessary (not requested) component of a package with a "Custom"
build-type is no longer built.
This caused breakage for Agda, and was undocumented.
The behaviour, despite breaking, is the desired one -- a package
shouldn't assume the executable will be built if only the library was
requested. This commit documents the breaking change to make sure it is
more visible.
In the meantime, we've also patched Agda to no longer expect the
executable to be built when only the library is installed.
Closes #9777 and #10235