produce a usable installation even after a build failure

CMake, by default, requires that all build artifacts have been correctly produced when installing.

It's possible to tell CMake that a build artifact is OPTIONAL, but it's not easy nor correct to change all invocations of the install command to use the OPTIONAL flag (we risk to deploy incomplete builds without noticing).

What we can do is to patch the CMake generated cmake_install.cmake files in the nightlies so that we can effectively turn all build artifacts to OPTIONAL in the special case of the nightly builds.