[RFC] Install only necessary dependencies for each build type.

Don't install unnecessary dependencies (e.g. jq, but not clang or doxygen for vimpatch report) to clean up the scripts and make builds a tiny bit faster.
Instead of putting a check in each `install_`/`setup_` function, how about wrapping the "setup steps" at this level?

```
function ci_install() {
  [[ "${CI}" == "true" ]] && return 0 || echo "Local build, not installing dependencies." ; return 1;
}

ci_install && (
  install_clang
  setup_neovim_deps
)
```

This `ci_install` (or whatever) function could be put in documentation.sh or common.sh.