Thanks to Leo for bringing up this missing feature on IRC.  I've been
meaning to implement this for awhile but lacked motivation. ;)

With this patch, you can freely compose packages whose inputs should be
in the environment with ad-hoc packages that should be added to the
environment directly.  For example, here's how to create a Guix
development environment that additionally has strace:

    guix environment guix --ad-hoc strace

The --load and --expression options are --ad-hoc aware as well.  Yay,
composability!