fix(nix): allow msrv test to have warnings

It is otherwise inconvenient or maybe in other cases even impossible to
have clippy pass with both the current Rust version as well as the MSRV
Rust version. (e.g. `clippy::missing_transmute_annotations` is not yet
known to our current MSRV.)

Signed-off-by: Thomas Mühlbacher <tmuehlbacher@posteo.net>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

flake.nix

@@ -283,7 +283,7 @@
                 // {
                   pname = "msrv";
                   inherit (common) cargoArtifacts;
-                  cargoClippyExtraArgs = "--all-targets --all-features -- --deny warnings";
+                  cargoClippyExtraArgs = "--all-targets --all-features";
                 }
               );
           };