fix: run 'cargo test' under 'nix flake check' with a non-debug profile

Summary: The Nix CI has been failing recently due to (what I assume is) disk
space issues. But only the `flake check` step is failing. Right now, `nix flake
check` runs the Cargo tests with the debug profile to help get more debug info,
which is even heftier in terms of debug info than the normal 'test' profile. For
reference, a single build of 'cargo test' in a clean working copy results in a
15 gigabyte `target/` directory.

Turn off the debug profile for `nix flake check`, which should hopefully stem
the bleeding a bit. I believe the 'test' profile should still have enough
symbols for backtraces, so panics should still be useful.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: Idde10ac15847a1ad1e6f4e48a2497eca

flake.nix

@@ -94,8 +94,6 @@
         program = "${self.packages.${system}.jujutsu}/bin/jj";
       };
       checks.jujutsu = self.packages.${system}.jujutsu.overrideAttrs ({ ... }: {
-        cargoBuildType = "debug";
-        cargoCheckType = "debug";
         preCheck = ''
           export RUST_BACKTRACE=1
         '';