Disable cargo-hack in CI
Disable cargo-hack in CI

Serde accidentally raised its MSRV to 1.51.0 in a patch release.  They
don't intent to fix it. Nix uses Serde via cargo-hack in CI.  Disable it
so we can publish a final release at 1.46.0.

asomers/nix,asomers/nix,nix-rust/nix,nix-rust/nix