GitHub actions: Use python 3.10.6 and ubuntu-22.04

I'm surprised because I don't remember this failing before, but
python version 3.10 is seemingly unavailable in GitHub runner using
"ubuntu-latest" (which resolves to Ubuntu 20.04 appparently), so let's
require ubuntu-22.04 and see.  And in addition, we had to change from
3.10.5 to 3.10.6 to make it run.

.github/workflows/buildcheck.yml

@@ -4,14 +4,14 @@ on: [push, pull_request]
 
 jobs:
   buildtest:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v2
 
       - name: Setup Python
         uses: actions/setup-python@v1
         with:
-          python-version: 3.10.5
+          python-version: 3.10.6
 
       - name: Install dependencies
         run: |