Add abitlity to skip binary installation

[wclr]

It would be useful to have the ability to skip purescript binary installation in certain scenarios.

For example, if the binary is supplied using another method, or just not needed. For example, in my case I have purescript as npm dependency in the project and it is used on host system, also packages declared in project are installed in docker container, but I don't run purs there, though the package should be is installed. But as I use alpine based images installations throws with error. So currently have to use a hacky way to avoid running purescirpt install.

The solution could be not to perform install operation if a env variable i.e. SKIP_INSTALL_PURESCRIPT is set. This is kind a common solution used by packages that require binary which may cause issues just as an example pupeeter uses PUPPETEER_SKIP_CHROMIUM_DOWNLOAD env for such case.