Fix execution of fixpack in projects with spaces in the CWD

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.