Downgrade Ubuntu version in CI

We require OTP < 24 in some of the test matrix, but GitHub's
ubuntu-latest = ubuntu-22.04, and erlef/setup-beam only supports OTP
24.2-27 on that version. So we have to lock it to ubuntu-20.04 to get
support for OTP 20.0 - 27.

https://github.com/actions/runner-images/blob/870d08d9cbff9a9a2b0e805c7e41e5238c184bf0/README.md#available-images
https://github.com/erlef/setup-beam/blob/0a541161e47ec43ccbd9510053c5f336ca76c2a2/README.md#compatibility-between-operating-system-and-erlangotp

.github/workflows/build.yml

@@ -46,7 +46,8 @@ jobs:
 
     name: Elixir ${{ matrix.elixir }} (OTP ${{ matrix.otp }})
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
+
 
     steps:
       - name: Checkout repository