CI Release Job: use exact versions available on ubuntu-20.04

The ubuntu-latest image maps to ubuntu-22.04, which does not have
Python 3.6 available.  The 20.04 based image does have all the
versions needed.

This pins the versions to the minor releases as specificed by the
image documentation.

Reference: https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2004-Readme.md#python
Signed-off-by: Cleber Rosa <crosa@redhat.com>

.github/workflows/release.yml

@@ -82,11 +82,11 @@ jobs:
 
   build-and-publish-eggs:
     name: Build eggs and publish them
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: release
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10.0, 3.11]
+        python-version: [3.6.15, 3.7.15, 3.8.16, 3.9.16, 3.10.9, 3.11.1]
       fail-fast: false
 
     steps: