Configure OS for erlef/setup-beam action

Resolves errors like the following:
```
Error: Tried to map a target OS from env. variable 'ImageOS' (got
undefined), but failed. If you're using a self-hosted runner, you should
set 'env': 'ImageOS': ... to one of the following: ['ubuntu18',
'ubuntu20', 'ubuntu22', 'ubuntu24', 'win19', 'win22']
```

Author: sgerrand

.github/workflows/ci.yml

@@ -82,6 +82,7 @@ jobs:
     container:
       image: ubuntu:focal-20250404
       env:
+        ImageOS: ubuntu20
         MIX_ENV: test
     strategy:
       fail-fast: false