Run the flake as part of CI

The update to the build script makes it so that the entire script exits
when a failure is encountered (and returns with that exit code).

.github/workflows/build.yml

@@ -10,3 +10,4 @@ jobs:
       - uses: cachix/install-nix-action@v19
       - run: nix build
       - run: nix flake check
+      - run: nix run

build.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 
+set -e
+
 # Following variables are set via sed in the buildPhase step
 PREFIX=""
 # Intentionally overriding PATH so supressing shellcheck