Fix for broken `hab pkg exec` with multiple `pkg_bin_dirs`
Fix for broken `hab pkg exec` with multiple `pkg_bin_dirs`

In, for example,
[the openssh core
plan](https://github.com/habitat-sh/core-plans/blob/993ac5705069c58c99abc2b4e9bb59c2f8b0fa32/openssh/plan.sh),
there are multiple `pkg_bin_dirs`, as an array:

    pkg_bin_dirs=(bin sbin libexec)

When running `RUST_LOG=debug hab pkg exec core/openssh ssh`, you get:

    ...
    PATH='/hab/pkgs/core/openssh/7.2p2/20161102155938/bin/hab/pkgs/core/openssh/7.2p2/20161102155938/sbin/hab/pkgs/core/openssh/7.2p2/20161102155938/libexec:/hab/pkgs/core/glibc/2.22/20160612063629/bin:/hab/pkgs/core/openssl/1.0.2j/20161102155324/bin'
    ✗✗✗
    ✗✗✗ `ssh' was not found on the filesystem or in PATH
    ✗✗✗

Note that the first item in the PATH is:

    /hab/pkgs/core/openssh/7.2p2/20161102155938/bin/hab/pkgs/core/openssh/7.2p2/20161102155938/sbin/hab/pkgs/core/openssh/7.2p2/20161102155938/libexec

but it's supposed to be:

    /hab/pkgs/core/openssh/7.2p2/20161102155938/bin:/hab/pkgs/core/openssh/7.2p2/20161102155938/sbin:/hab/pkgs/core/openssh/7.2p2/20161102155938/libexec

This adds a missing push of a `:` to make sure packages that have
multiple `pkg_bin_dirs` get their paths set correctly.

Signed-off-by: Nathan L Smith <2b5c240e6abd88e71ffc225b0459016e4cba9bda@chef.io>

rsertelon/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat