Currently the spawn command takes any number of arguments and naively
joins them together with spaces before passing them as the single
argument of `/bin/sh -c`. This however produces unexpected results as
soon as shell quoting gets involved in the arguments passed to spawn.
For example, running
riverctl spawn foo "bar baz"
will execute `/bin/sh -c "foo bar baz"`, unexpectedly splitting bar and
baz into separate arguments. To avoid this confusion, make the spawn
command take only a single argument, forcing the user to quote properly
to spawn multi-argument commands.
b7e15a8ef6