osbuild currently throws an error when not passing a build environment
on the command line, because the runner is unset. This is annoying on
hosts which only need a runner set, but no build pipeline.
To simplify running osbuild in this common case, introduce
`org.osbuild.host`, which is a runner that is defined to work on the
host that osbuild is installed on. Use this runner by default and
include a symlink to the right runner in the Fedora and RHEL packages.
Also add `runners/org.osbuild.host` to `.gitignore`, so that developers
can set the symlink when running osbuild from the source directory.
Fixes#171
f0a7b2261e