CI: use cargo hack to test --feature-powerset

[tarcieri]

No description provided.

[tarcieri]

Huh, guess --feature-powerset doesn't include optional dependencies. TIL

[newpavlov]

We could define the features explicitly:

[depednencies]
opt_dep = { version = "*", optional = true }

[features]
opt_dep = ["dep:opt_dep"]

[tarcieri]

@newpavlov yeah, that's another solution. I ended up using the --optional-deps flag FWIW, though it would be nice for it to be automatic