ci; bump ubuntu ver for picosystem blit build

...which ended up modifying the picovision build due to diff weirdness

.github/workflows/cmake.yml

diff --git a/.github/workflows/cmake.yml b/.github/workflows/cmake.yml
index 0c29ee4..df4f19e 100644
--- a/.github/workflows/cmake.yml
+++ b/.github/workflows/cmake.yml
@@ -48,7 +48,7 @@
             cmake-args: -D32BLIT_DIR=$GITHUB_WORKSPACE -DPICO_SDK_PATH=$GITHUB_WORKSPACE/pico-sdk -DPICO_BOARD=pimoroni_picosystem -DBLIT_EXECUTABLE_PICO_BLIT=1 -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache
             apt-packages: ccache gcc-arm-none-eabi libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib python3-setuptools
 
-          - os: ubuntu-20.04
+          - os: ubuntu-22.04
             pico-sdk: true
             name: PicoVision
             cache-key: picovision