CI: Use an older version of Linux to satisfy the MicroPython build

Building MicroPython v1.20.0 failed:

  ../py/stackctrl.c:32:32: error: storing the address of local variable
  ‘stack_dummy’ in ‘mp_state_ctx.thread.stack_top’
  [-Werror=dangling-pointer=]

Author: amotl

.github/workflows/tests.yml

@@ -87,7 +87,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ['ubuntu-latest']
+        os: ['ubuntu-22.04']
         micropython-version: [
           'v1.20.0',
           'v1.24.0',