fix flaky cf files test

We were testing against an app with two instances. `gcf` only waits for
one index to report RUNNING, but `gcf files` always checks index 0. So,
if index 1 came up before 0, `gcf files` would reliably fail.

There is a bug in `gcf` here: `gcf files` should grab files of only a
running instance, potentially a random one, to fit the abstraction level
of `gcf push` (specific instances don't matter).

[finishes #60908262]