installer/cmd/tectonic/main: List 'full' first among 'install' subcommands
installer/cmd/tectonic/main: List 'full' first among 'install' subcommands

2b82fbe6 (installer: Integrate multistep cli with configuration,
2018-02-16, coreos/tectonic-installer#2960) shifted it into the middle
of the smaller install subcommands.  But because:

* those smaller subcommands are, with the exception of newtls, subsets
  of full, and
* full is the default action,

I think it makes more sense to move it back to the top.

derekhiggins/installer,derekhiggins/installer