release-26.2: roachtest: ssh_problem failed

**Note:** This build has runtime assertions enabled. If the same failure was hit in a run without assertions enabled, there should be a similar failure without this message. If there isn't one, then this failure is likely due to an assertion violation or (assertion) timeout.

roachtest.ssh_problem [failed](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Nightlies_RoachtestNightlyGceBazel/21273442?buildTab=log) with [artifacts](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Nightlies_RoachtestNightlyGceBazel/21273442?buildTab=artifacts#/backup/nodeShutdown/worker/n4cpu4) on release-26.2 @ [cc34a3eefe9f9548beaa55ced23d3503332bac20](https://github.com/cockroachdb/cockroach/commits/cc34a3eefe9f9548beaa55ced23d3503332bac20):

```
test backup/nodeShutdown/worker/n4cpu4 failed: (test_runner.go:998).func4: cluster.PutE: put "/go/src/github.com/cockroachdb/cockroach/bin/cockroach-ea.linux-amd64" failed: error persisted after 3 attempts: TRANSIENT_ERROR(ssh_problem): ~ scp -r -C -o StrictHostKeyChecking=no -o ConnectTimeout=10 -R -A -i /home/roach/.ssh/id_rsa ubuntu@34.26.25.214:./cockroach ubuntu@34.148.37.14:./cockroach
Connection to 34.148.37.14 closed by remote host.
lost connection: exit status 1 [owner=test-eng]
```

Parameters:
- arch=amd64
- cloud=gce
- coverageBuild=false
- cpu=4
- diskCount=0
- encrypted=false
- runtimeAssertionsBuild=true
Help

See: [roachtest README](https://github.com/cockroachdb/cockroach/blob/master/pkg/cmd/roachtest/README.md)

See: [How To Investigate \(internal\)](https://cockroachlabs.atlassian.net/l/c/SSSBr8c7)

See: [Grafana](https://go.crdb.dev/roachtest-grafana/teamcity-21273442/backup-nodeshutdown-worker-n4cpu4/-62135596800000/-62135596680000)

/cc @cockroachdb/test-eng

[This test on roachdash](https://roachdash.crdb.dev/?filter=status:open%20t:.*ssh_problem.*&sort=title+created&display=lastcommented+project) | [Improve this report!](https://github.com/cockroachdb/cockroach/tree/master/pkg/cmd/bazci/githubpost/issues)

Jira issue: CRDB-61761

Start with the failure in test_runner.go:998, the linked TeamCity build log and artifacts, and the roachtest README. Trace the backup/nodeShutdown/worker test's cluster.Put failure and SSH/scp connection closure; done means identifying the cause and making this roachtest pass reliably.

Written by the indexing model from the issue text.