fix: ark-std Cargo.toml path

[hantastic3]

Update incorrect ark-std path in various Cargo.toml files

[SadPencil]

Did you get an error or warning when build the mpc-snarks cargo?
I tested the difference applying/not applying your fix.

Excluding your fix, I would get

error: duplicate key `path`
  --> ../curves/mnt6_298/Cargo.toml:18:12
   |
18 | ark-std = { path = "../util",  path = "../util", default-features = false }
   |            ^
   |

it seems like an error but the build process succeed in the end so it is more likely a warning?