fix(ci): keep snap canary sandbox name within limit

[drew]

Summary

Fix the Release Canary failure in the Ubuntu Snap with system Docker lane. The old sandbox name exceeded the gateway's 19-character limit, so sandbox creation returned InvalidArgument before exercising the runtime.

Related Issue

No issue required: this is an obvious localized CI workflow bug. The failure is visible in Release Canary run 36290090668.

Changes

• Use rc-${GITHUB_RUN_ID} for the Snap lane's sandbox name, consistent with the other canary lanes and within the gateway limit.

Testing

• mise run pre-commit passes.
• Release Canary branch run 36340894904 passes all six jobs, including Snap sandbox create, exec, and delete.
• mise run ci passes locally. It fails in unrelated Go gateway tests because this workstation's system gateway configuration adds an unexpected default gateway to their fixtures.
• Unit tests added/updated (not applicable to this one-line workflow fix).
• E2E tests added/updated (covered by the branch canary).

Checklist

• Follows Conventional Commits.
• Commit is signed off (DCO).
• Architecture docs updated (not applicable).