c0879983

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Use dontInstall instead of a no-op installPhase

The coverage and report derivations populate $out directly in buildPhase, so
the install phase is unnecessary.  Replace the no-op
`installPhase = "runHook preInstall; runHook postInstall"` (which only existed
to suppress stdenv's default `make install`) with the idiomatic
`dontInstall = true`.