ci(coverage): allow 20 minutes per Go test package

[XuPeng-SH]

Cause

MatrixOne Coverage wraps the whole Go test phase with a 3600-second deadline, but does not set go test -timeout. Go therefore still applies its default 10-minute deadline to each package. In the failed matrixorigin/matrixone#29368 run, pkg/tests/issues/isolated had completed 11 tests in 574.35 seconds before its final test began, then hit the package alarm at 600 seconds. The final test's name in the panic is incidental to the aggregate package runtime.

Change

Set a finite 20-minute Go package deadline below the existing 60-minute whole-suite deadline. In the observed run this package began about 11 minutes into the phase, so a full 20-minute package run would still leave about 29 minutes for remaining work. This corrects the nested budget mismatch while retaining Go's per-package stack dump for real stalls.

Related: matrixorigin/matrixone#29368. The separate CN shutdown-context observation and slow fixture optimization remain tracked there; this PR does not claim to fix those paths.

Validation

• Reviewed the raw failed Coverage JSON timeline: 301.61 seconds in TestAutoIDCacheRestartAndDisabledNode, 157.35 seconds in TestIssue28234ExceptAllCluster, and 115.39 seconds in the other nine passed tests before the package alarm.
• Ruby YAML parser accepted the changed workflow; git diff --check passed.
• Independent design and final diff reviews found no blocker.

Full reusable-workflow validation requires a MatrixOne PR run after this CI PR is merged. Revert this three-line change to restore the prior package deadline if needed.