Fix execution of fixpack in projects with spaces in the CWD

[creynders]

Hi there,

Fixpack couldn't get executed in projects with spaces in the path, I fixed it by surrounding the path with double quotes.

I added an additional optimization: instead of executing node <...>/node_modules/fixpack/fixpack it's better to execute the bin file in node_modules/.bin directly. Obviously ATM this is the same thing since .bin/fixpack file is a symlink, but the advantage is that should fixpack change their directory structure (and dump the fixpack file in a lib dir for instance) then you won't have to update your code, otherwise you would.

However if you don't like the optimization, just cherry-pick the first commit to fix the execution in paths with spaces.

[bahmutov]

Thanks for fixing this. It would be even nicer to be able to use fixpack as a module or at least grab the bin path from fixpack's package.json