buildRustPackage, fetchcargo: optionally use vendor config from cargo-vendor

By setting useRealVendorConfig explicitly to true, the actual (slightly
modified) config generated by cargo-vendor is used.

This solves a problem, where the static vendor config in
pkgs/build-support/rust/default.nix would not sufficiently replace all
crates Cargo is looking for.

As useRealVendorConfig (and writeVendorConfig in fetchcargo) default to
false, there should be no breakage in existing cargoSha256 hashes.

Nethertheless, imho using this new feature should become standard. A
possible deprecation path could be:

- introduce this patch
- set useRealVendorConfig explicitly to false whereever cargoSha256 is
  set but migration is not wanted yet.
- after some time, let writeVendorConfig default to true
- when useRealVendorConfig is true everywhere cargoSha256 is set and
  enough time is passed, `assert cargoVendorDir == null ->
  useRealVendorConfig;`, remove old behaviour
- after some time, remove all appearences of useRealVendorConfig and the
  parameter itself