Gateway errors fetching builds.txt still cause intermittent failures

#### The bug

`erlef/setup-beam@v1` still intermittently fails while fetching `builds.txt` from `https://builds.hex.pm` with `Gateway Error`. This was observed after the retry change from #304 was merged.

Two jobs in the same workflow failed this way:

- Gleam: https://github.com/adamtheturtle/literalizer/actions/runs/34466017172/job/102834669738
- Elixir: https://github.com/adamtheturtle/literalizer/actions/runs/34466017172/job/102834669824

#### Software versions

- `setup-beam`: `@v1`
- runner: GitHub-hosted Ubuntu 24.04
- OTP: 27

#### How to replicate

Run `erlef/setup-beam@v1` repeatedly on GitHub-hosted Ubuntu. The intermittent failure is:

```text
Action fetch /builds/otp/amd64/ubuntu-24.04/builds.txt failed for mirror https://builds.hex.pm, with Error: Gateway Error
Error: Could not fetch /builds/otp/amd64/ubuntu-24.04/builds.txt from any hex.pm mirror
```

#### Expected behaviour

Transient gateway failures should be retried sufficiently, or a cached strict-version build should avoid the `builds.txt` request.

#### Additional context

This appears related to #260, which was closed by #304, but demonstrates that the failure mode remains.