flake: Fix devshell for darwin hosts (#5368)

Runs of `cargo build` would fail with `ld: framework not found
CoreFoundation` inside a devshell (`nix develop`) on a Darwin host.

This is fixed with two changes:

* Including the `CoreFoundation` package within the devshell's
  derivation.
* Switching from `value` to `eval` so that we append to the `RUSTFLAGS`
  environment variable rather than overwriting it. `numtide/devshell`
  already exposes the `Library/Frameworks` path within the devshell
  derivation but it was being overwritten by the manual setting of
  `RUSTFLAGS` in `env`.

0 files changed, 0 insertions, 0 deletions