workflows/build: stop building for x86_64-darwin

This will apply to the stable release branches too, but that seems
unavoidable: we use `nixpkgs-unstable` in `ci/pinned.json`, which
means that the development shell for stable release branches is based
on the unstable branch. Once we drop support for a system on the
unstable branch, it can no longer be used for Nixpkgs development on
the next pin, even when targeting already‐released stable branches.

Since Nixpkgs contributors would generally be expected to be less
likely to be using deprecated platforms, and we usually operate with
a backport workflow where changes target the unstable branch first,
this is probably acceptable. A potential alternative would be to pin
the stable branch for CI instead.

.github/workflows/build.yml

@@ -34,21 +34,18 @@ jobs:
       matrix:
         include:
           - runner: ubuntu-24.04
-            name: x86_64-linux
-            systems: x86_64-linux
+            system: x86_64-linux
             builds: [shell, manual-nixos, lib-tests, tarball]
             desc: shell, docs, lib, tarball
           - runner: ubuntu-24.04-arm
-            name: aarch64-linux
-            systems: aarch64-linux
+            system: aarch64-linux
             builds: [shell, manual-nixos, manual-nixpkgs]
             desc: shell, docs
           - runner: macos-14
-            name: darwin
-            systems: aarch64-darwin x86_64-darwin
+            system: aarch64-darwin
             builds: [shell]
             desc: shell
-    name: '${{ matrix.name }}: ${{ matrix.desc }}'
+    name: '${{ matrix.system }}: ${{ matrix.desc }}'
     runs-on: ${{ matrix.runner }}
     timeout-minutes: 60
     steps:
@@ -80,7 +77,9 @@ jobs:
 
       - name: Build shell
         if: contains(matrix.builds, 'shell')
-        run: echo "${{ matrix.systems }}" | xargs -n1 nix-build-uncached nixpkgs/untrusted/ci --arg nixpkgs ./nixpkgs/untrusted-pinned -A shell --argstr system
+        env:
+          system: ${{ matrix.system }}
+        run: nix-build-uncached nixpkgs/untrusted/ci --arg nixpkgs ./nixpkgs/untrusted-pinned -A shell --argstr system "$system"
 
       - name: Build NixOS manual
         if: |
@@ -108,5 +107,5 @@ jobs:
           contains(fromJSON(inputs.baseBranch).type, 'primary')
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          name: ${{ inputs.artifact-prefix }}nixos-manual-${{ matrix.name }}
+          name: ${{ inputs.artifact-prefix }}nixos-manual-${{ matrix.system }}
           path: nixos-manual