bootstap.sh: post-install message doesn't make sense when using --global

[taavi]

When using the bootstrap.sh script with --global then the message displayed after a successful installation will look like this:

The 'cabal' program has been installed in /usr/local/bin/
You should either add /usr/local/bin to your PATH
or copy the cabal program to a directory that is on your PATH.

This is probably redundant since most users will have /usr/local/bin on their path.