Improve message output for multiple EOL platforms (#2859)

When multiple tasks in the same workflow use the same EOL platform, it
gets printed many times in a row, which is not helpful. See
https://buildkite.com/bazel/buildfarm-farmer/builds/13552#01a0a569-902e-44ae-a40e-9d2818fdacb3.

```
WARNING!! You are using end-of-life platforms: ubuntu2004, ubuntu2004,
ubuntu2004, ubuntu2004, ubuntu2004, ubuntu2004, ubuntu2004, ubuntu2004,
ubuntu2004
```

This PR de-dupes the list and makes the output less spammy.