Bytecode PR check in solc-bin fails on external PRs: `unknown revision or path not in the working tree`

The bytecode PR check in solc-bin [assumes that PR branches are always available in the repo](https://github.com/ethereum/solc-bin/blob/199db16cc9b2af20c04536ff0b1c7bba21a874bf/.github/workflows/t-bytecode-compare.yml#L66-L71). This results in CI failures on external PRs, for example [5140692597](https://github.com/ethereum/solc-bin/actions/runs/3158544890/jobs/5140692597) (https://github.com/ethereum/solc-bin/pull/123):
```
Commit range: origin/gh-pages..origin/ci-to-nightly
fatal: ambiguous argument 'origin/ci-to-nightly': unknown revision or path not in the working tree.
```

The code needs to be adjusted to handle external PRs as well, e.g. by creating a local tracking branch in the checkout.

Start with .github/workflows/t-bytecode-compare.yml around lines 66–71 and inspect how the checkout exposes PR branches. Reproduce the bytecode PR check with an external PR, then verify that the commit range resolves and the workflow completes without the unknown-revision error.

Written by the indexing model from the issue text.