Commit d028ea5b16 introduced bug when introducing the `store`
argument to `Stage.run`, instead of passing `var=var`, i.e.
`var` is being passed as keyword argument, it is now being
passed as a positional one. Since the `path=/run/osbuild`
keyword argument comes before the `var=/var/tmp` argument,
`var` is now being passed as `path` instead of var.
Since `var` is always being passed in throughout the entire
codebase, make it a positional argument, and move it before
`path`.
Adapt the tests to pass `var` as positional argument.
749f59cc9a