Add timeout to TPM SSH workflow to prevent multi-hour hangs

[aidangarske]

The TPM SSH Test workflow has no job timeout, so it inherits GitHub's 6-hour default. This run hung for ~2 hours before being cancelled.

Changes:

• Add timeout-minutes: 20 at the job level so any hung matrix leg is killed instead of running for hours. The job normally finishes in a few minutes.
• Wrap the client public-key auth step in timeout 20. Unlike the host-key test (already wrapped in timeout 20), the bare client invocation had no timeout and is the most likely indefinite-hang source — this makes it fail fast and cleanly rather than consuming the full job budget.