workflows/build: build shells on a single darwin runner only

Because we have Rosetta, we can use a single darwin runner to build both
shells. Most time is spent for downloading Nix and checking out Nixpkgs
anyway and this can be saved if only run once.

Also, this prepares for the deprecation of the old macos-13 runners,
which were based on x86_64 CPUs in September 2025:
https://github.blog/changelog/2025-07-11-upcoming-changes-to-macos-hosted-runners-macos-latest-migration-and-xcode-support-policy-updates/

.github/workflows/build.yml

@@ -26,22 +26,21 @@ jobs:
       matrix:
         include:
           - runner: ubuntu-24.04
-            system: x86_64-linux
+            name: x86_64-linux
+            systems: x86_64-linux
             builds: [shell, manual-nixos, lib-tests, tarball]
             desc: shell, docs, lib, tarball
           - runner: ubuntu-24.04-arm
-            system: aarch64-linux
+            name: aarch64-linux
+            systems: aarch64-linux
             builds: [shell, manual-nixos, manual-nixpkgs, manual-nixpkgs-tests]
             desc: shell, docs
-          - runner: macos-13
-            system: x86_64-darwin
-            builds: [shell]
-            desc: shell
           - runner: macos-14
-            system: aarch64-darwin
+            name: darwin
+            systems: aarch64-darwin x86_64-darwin
             builds: [shell]
             desc: shell
-    name: '${{ matrix.system }}: ${{ matrix.desc }}'
+    name: '${{ matrix.name }}: ${{ matrix.desc }}'
     runs-on: ${{ matrix.runner }}
     timeout-minutes: 60
     steps:
@@ -70,13 +69,13 @@ jobs:
 
       - name: Build shell
         if: contains(matrix.builds, 'shell')
-        run: nix-build-uncached untrusted/ci --arg nixpkgs ./pinned -A shell
+        run: echo "${{ matrix.systems }}" | xargs -n1 nix-build-uncached untrusted/ci --arg nixpkgs ./pinned -A shell --argstr system
 
       - name: Build NixOS manual
         if: |
           contains(matrix.builds, 'manual-nixos') && !cancelled() &&
           contains(fromJSON(inputs.baseBranch).type, 'primary')
-        run: nix-build-uncached untrusted/ci --arg nixpkgs ./pinned -A manual-nixos --argstr system ${{ matrix.system }} --out-link nixos-manual
+        run: nix-build-uncached untrusted/ci --arg nixpkgs ./pinned -A manual-nixos --out-link nixos-manual
 
       - name: Build Nixpkgs manual
         if: contains(matrix.builds, 'manual-nixpkgs') && !cancelled()
@@ -100,5 +99,5 @@ jobs:
           contains(fromJSON(inputs.baseBranch).type, 'primary')
         uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         with:
-          name: nixos-manual-${{ matrix.system }}
+          name: nixos-manual-${{ matrix.name }}
           path: nixos-manual