github/workflows: Use ubuntu-20.04 when python2 is required.

Python 2 is no longer included in the latest Ubuntu 22.04.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/mpy_format.yml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 # use 20.04 to get python2
     steps:
     - uses: actions/checkout@v3
     - name: Install packages

.github/workflows/ports_unix.yml

@@ -98,7 +98,7 @@ jobs:
       run: tests/run-tests.py --print-failures
 
   nanbox:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 # use 20.04 to get python2
     steps:
     - uses: actions/checkout@v3
     - name: Install packages