Fix build (real).

.github/workflows/build.yml

@@ -31,6 +31,7 @@ jobs:
     - name: Linux Toolchain (arm-cross)
       if: ${{ matrix.platform.os == 'ubuntu-latest' && matrix.toolchain.cross }}
       run: |
+        sudo apt-get update
         sudo apt-get upgrade
         sudo apt install -y gcc-arm-none-eabi
         sudo apt-get install -y ninja-build