Fix Github Action CI tests on Postgres cluster for versions <= 11

[nicolas-fricke]

Around April 15th the CI setup testing against Postgres started failing. This failure was unrelated to any code changes and started happening during the setup steps of the job, before running any code from the repository.

The error seems to originate from the ankane/setup-postgres step and seems related to it not being able to download and install the correct postgresql package via apt:

Run ankane/setup-postgres@v1
sudo pg_dropcluster 13 main
sudo apt-get update -o Dir::Etc::sourcelist="sources.list.d/pgdg.list" -o Dir::Etc::sourceparts="-" -o APT::Get::List-Cleanup="0"
Reading package lists...
W: Unable to read /etc/apt/-/ - DirectoryExists (2: No such file or directory)
W: Unable to read /etc/apt/sources.list.d/pgdg.list - RealFileExists (2: No such file or directory)
sudo apt-get install postgresql-11
Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package postgresql-11
child_process.js:660
    throw err;
    ^

Error: Command failed: sudo apt-get install postgresql-11
    at checkExecSyncError (child_process.js:621:11)
    at execSync (child_process.js:657:15)
    at run (/home/runner/work/_actions/ankane/setup-postgres/v1/index.js:7:3)
    at Object.<anonymous> (/home/runner/work/_actions/ankane/setup-postgres/v1/index.js:96:5)
    at Module._compile (internal/modules/cjs/loader.js:959:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:995:10)
    at Module.load (internal/modules/cjs/loader.js:815:32)
    at Function.Module._load (internal/modules/cjs/loader.js:727:14)
    at Function.Module.runMain (internal/modules/cjs/loader.js:1047:10)
    at internal/main/run_main_module.js:17:11 {
  status: 100,
  signal: null,
  output: [ null, null, null ],
  pid: 2392,
  stdout: null,
  stderr: null
}

This is an example of the error occuring.