It's confusing that the Makefile uses a different name than CMake.

This is intended to be used with source introspection tools like
clangd, where it would useful to regenerate headers and source files,
which introspection results depend on, without making a full
rebuild which takes much longer time than just generating headers.

Without this, the CI_TARGET=lint travis job cant't find the cached deps
(in $HOME/nvim-deps), nor can it update the cache.

This will allow users to use AppImageUpdate to update their AppImage.
It requires publishing the created zsync file alongside the appimage
file for the releases.

scripts/genappimage.sh produces an executable:
    nvim-${NVIM_VERSION}-glibc${GLIBC_VERSION}-${ARCHITECTURE}.AppImage

Closes #6083

This also removes LINT_FILE environment variable, other then that functionality 
is kept. It is expected that developers needing partial linting will use `make 
lint`, touching interesting file before (if not done already by writing to 
them).