magrathea: mg run no longer works without arguments

Using it without arguments:

tazjin@tverskoy /depot (public-inbox)> mg run //tools/fetch-depot-inbox
[mg] error: usage: mg run [<target>] [-- <arguments>] (hint: use "--" to separate the `mg run [<target>]` invocation from the arguments you're passing to the built executable)

but with an empty set of args (i.e. just --):

tazjin@tverskoy /depot (public-inbox) [1]> mg run //tools/fetch-depot-inbox --
[mg] building target //tools/fetch-depot-inbox
[mg] running target //tools/fetch-depot-inbox
hello

Not sure what happened.