Bootstrap: Show 'system requirements' before questions

[dfordivam]

fixes #1256

This is how it looks now

Welcome to Haskell!

This script can download and install the following binaries:
  * ghcup - The Haskell toolchain installer
  * ghc   - The Glasgow Haskell Compiler
  * cabal - The Cabal build tool for managing Haskell software
  * stack - A cross-platform program for developing Haskell projects (similar to cabal)
  * hls   - (optional) A language server for developers to integrate with their editor/IDE

ghcup installs only into the following directory,
which can be removed anytime:
  /home/divam/.ghcup

Press ENTER to proceed or ctrl-c to abort.
Note that this script can be re-run at any given time.

[ Info  ] downloading: https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-0.0.9.yaml as file /home/divam/.ghcup/cache/ghcup-0.0.9.yaml
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
[ Info  ] downloading: https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-prereleases-0.0.9.yaml as file /home/divam/.ghcup/cache/ghcup-prereleases-0.0.9.yaml
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
[ Info  ] downloading: https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-cross-0.0.9.yaml as file /home/divam/.ghcup/cache/ghcup-cross-0.0.9.yaml
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
[ Info  ] Upgrading GHCup...
[ Warn  ] No GHCup update available

System requirements 
  Please ensure the following distro packages are installed before continuing (you can exit ghcup and return at any time): build-essential curl libffi-dev libffi8 libgmp-dev libgmp10 libncurses-dev pkg-config

Press ENTER to proceed or ctrl-c to abort.