ci: restore GitHub Actions

1. Fix the dev/ci.sh script so it properly fails on error. And use nix-eval-jobs from nixpkgs input.
2. Build x86_64-linux on ubuntu-latest

.github/workflows/nix.yml

@@ -9,12 +9,8 @@ on:
   workflow_dispatch:
 jobs:
   build:
-    strategy:
-      matrix:
-        system:
-          - aarch64-darwin
-          - x86_64-linux
-    runs-on: [nix]
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - run: ./dev/ci.sh --argstr system ${{ matrix.system }}
+      - uses: cachix/install-nix-action@v23
+      - run: ./dev/ci.sh

dev/ci.sh

@@ -26,7 +26,9 @@ nix flake lock "$(dirname "$0")" --update-input srvos
 
 error=0
 
-for job in $(nix-eval-jobs "${args[@]}" | jq -r '. | @base64'); do
+output=$(nix run --inputs-from . "nixpkgs#nix-eval-jobs" -- "${args[@]}")
+
+for job in $(echo "$output" | jq -r '. | @base64'); do
   job=$(echo "$job" | base64 -d)
   attr=$(echo "$job" | jq -r .attr)
   echo "### $attr"