Fix nix-matrix job status

The nix-matrix job was reported as successful when in reality the nix
eval command had a non-zero exit code and failed to generate the matrix.
By splitting the echo and nix eval commands up into two lines the exit
code is properly detected and fails the nix-matrix job.

.github/workflows/nix-github-actions.yml

@@ -18,7 +18,8 @@ jobs:
         name: Generate Nix Matrix
         run: |
           set -Eeu
-          echo "matrix=$(nix eval --json '.#githubActions.matrix')" >> "$GITHUB_OUTPUT"
+          matrix="$(nix eval --json '.#githubActions.matrix')"
+          echo "matrix=$matrix" >> "$GITHUB_OUTPUT"
 
   nix-build:
     needs: nix-matrix