fix(github): run 'nix build' as part of CI, too

Summary: While I was developing #1846, I found a case where `nix flake check`
succeeded but `nix build` on the jujutsu expression failed, which was confusing
to me. If it had been merged, it would have broke the ability to use Nix to
install things.

Let's try to attack this, and punch a parallel job into the build matrix;
hopefully running the two jobs concurrently can help catch this without
making build time much worse.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: If2520d69492fa6abe499c1cb1d51d6e1

.github/workflows/nix-linux.yml

@@ -10,7 +10,12 @@ permissions: read-all
 
 jobs:
   nix:
-    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ ubuntu-latest ]
+        mode: [ "flake check", "build" ]
+    runs-on: ${{ matrix.os }}
     name: nix-build
     timeout-minutes: 20
     steps:
@@ -21,4 +26,4 @@ jobs:
         with:
           extra_nix_config: |
             experimental-features = nix-command flakes
-      - run: nix flake check --print-build-logs --show-trace
+      - run: nix ${{ matrix.mode }} --print-build-logs --show-trace