Update vars to secrets in awsmegatest action

[FriederikeHanssen]

Severity: medium (operational) — ${{ vars.X }} was migrated to ${{ secrets.X }} for TOWER_WORKSPACE_ID, TOWER_COMPUTE_ENV, and TOWER_BUCKET_AWS (also at lines 106 and 108). Two things worth confirming before merge:

1. Existing repo vars entries don't auto-promote to secrets — a repo admin needs to populate the same names under Settings → Secrets, otherwise seqeralabs/action-tower-launch will fire with empty values and the failure mode isn't obvious.
2. Workspace ID and compute-env name aren't really sensitive (they appear in run URLs and the launch step's outputs); the masking won't change much in practice. If this was intentional uplift to align with another repo's conventions, fine — worth a one-line note in the changelog entry.

Originally posted by @pinin4fjords in #2168 (comment)