fix(ci): skip remote sccache config for fork PRs

[drew]

Summary

Avoid exporting SCCACHE_MEMCACHED_ENDPOINT at workflow scope in branch checks so fork PRs do not pass an empty memcached endpoint into sccache.

Related Issue

N/A

Changes

• Removed the workflow-level SCCACHE_MEMCACHED_ENDPOINT export from branch checks
• Added a Rust job step that writes SCCACHE_MEMCACHED_ENDPOINT to $GITHUB_ENV only when the repository variable is present
• Preserved shared remote sccache for internal PRs while allowing fork PRs to run without it

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)