Use foundry releases on external tests

The external test jobs have been consistently failing (for example, see: https://app.circleci.com/pipelines/github/argotorg/solidity/41064/workflows/804d4a9f-91fa-40e4-a2d1-8e6e997fd752/jobs/1914135 and https://app.circleci.com/jobs/github/argotorg/solidity/1911617). The errors are related to the `install_foundry` step, which attempts to download the [latest nightly builds](https://github.com/argotorg/solidity/blob/c8ccc6c25d92a8f475ff135db92d1b09a4d264bd/.circleci/config.yml#L138) from Foundry on each run.

The job uses a GitHub token with read permissions to avoid hitting rate limits for unauthorized requests, but due to the frequency of the downloads, occasional failures still occur with 503 or 403.

This setup was initially implemented to test for breaking changes while Foundry did not have stable releases (only pre-releases). However, since Foundry now appears to have proper releases, we should modify the logic to use stable releases instead of nightly builds.

Read .circleci/config.yml around line 138 and trace the install_foundry step used by the external test jobs. Check Foundry's stable release mechanism, then run the affected external tests to verify they use a stable release rather than nightly builds and no longer fail from repeated download requests.

Written by the indexing model from the issue text.