ci: correct toolchain matrix args

.github/workflows/build.yaml

@@ -11,20 +11,23 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        toolchain: [1.88.0]
         include:
           - os: ubuntu-latest
             target: x86_64-unknown-linux-gnu
             artifact_name: pacman
+            toolchain: 1.88.0
           - os: macos-13
             target: x86_64-apple-darwin
             artifact_name: pacman
+            toolchain: 1.88.0
           - os: macos-latest
             target: aarch64-apple-darwin
             artifact_name: pacman
+            toolchain: 1.88.0
           - os: windows-latest
             target: x86_64-pc-windows-gnu
             artifact_name: pacman.exe
+            toolchain: 1.88.0
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout