[studio] Fix building the Windows Docker image when installing pkgs.
[studio] Fix building the Windows Docker image when installing pkgs.

Prior to this fix, the `core/windows-service --channel stable` item was
being passed to `hab pkg install` as a single argument, rather than
multiple arguments. iex ftw.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat