Fix clippy warnings or disable clippy in Nix

[srid]

A full Nix build (via nix run github:srid/nixci build) fails, due to clippy warnings:

If clippy is not used, I'd get rid of this:

https://github.com/juspay/superposition/blob/761a69b397624b6fc03413d65289c3573f9f578e/rust.nix#L49

To reproduce,

❯ nix flake check
warning: unknown flake output 'haskellFlakeProjectModules'
error: builder for '/nix/store/8qc30sswzi47pagn915asnvdk9ri9j7p-superposition-clippy-0.0.1.drv' failed with exit code 101;
       last 10 log lines:
       >     |
       > 107 |                 s.to_string()
       >     |                  ^^^^^^^^^^^^ help: remove this
       >     |
       >     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_format_args
       >     = note: `-D clippy::to-string-in-format-args` implied by `-D warnings`
       >     = help: to override `-D warnings` add `#[allow(clippy::to_string_in_format_args)]`
       >
       > error: could not compile `service_utils` (lib) due to 21 previous errors
       > error: could not compile `service_utils` (lib test) due to 21 previous errors
       For full logs, run 'nix log /nix/store/8qc30sswzi47pagn915asnvdk9ri9j7p-superposition-clippy-0.0.1.drv'.