default pinpointed macos-13, update nix action

This resolves an error with the darwin based builds, where the install
fails. Pinpointing the macos version will prevent random failrue in
the future --- now we have to opt-in to potential breaking changes when
a new macos release is added to the GitHub Actions runners.

relevant error message:

```console
...
---- Reminders -----------------------------------------------------------------
[ 1 ]
Nix won't work in active shell sessions until you restart them.

Could not set environment: 150: Operation not permitted while System Integrity Protection is engaged
Error: Process completed with exit code 150.
```

fixes #100

.github/workflows/qc.yaml

@@ -82,7 +82,10 @@ jobs:
           key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
       - name: Install libsodium
         run: sudo apt-get install -y libsodium-dev
-      - run: cargo test
+        # liboqs requires quite a lot of stack memory, thus we adjust
+        # the default stack size picked for new threads (which is used
+        # by `cargo test`) to be _big enough_. Setting it to 8 MiB
+      - run: RUST_MIN_STACK=8388608 cargo test
 
   cargo-test-nix-devshell-x86_64-linux:
     runs-on: