jhbuild/commands: Disable interspersed args

The option parser previously did not stop parsing options after the first positional argument. This would result in option parsing errors when trying to pass arguments to the executable passed to jhbuild run. For example jhbuild run --in-builddir=mutter test --version results in jhbuild: error: no such option: --version.

Disable this behavior with disable_interspersed_args(). This should be fine to enable globally because according to the help description of all commands the positional arguments are always last.