Renovate pipeline task hangs ~15 minutes after Renovate exits with an error

When Renovate encounters an `unhandledRejection` error during `postUpgradeTasks` execution (e.g. a missing tool in the container), the "Run Renovate" pipeline task hangs for ~15 minutes between the Renovate process exiting and the bash wrapper script completing.

### Observed behavior

Build [#2929718](https://dev.azure.com/dnceng/internal/_build/results?buildId=2929718) (`dotnet-docker-renovate` pipeline):

- **15:20:46** — Renovate logs its final output and exits with code 1
- **15:35:46** — The bash task reports `Bash exited with code '1'` — **15 minutes later**

During the gap, the only log output is periodic `Agent environment resources` debug lines. No useful work is being done.

Copilot's root cause hypothesis

The pipeline template runs Renovate with this wrapper:

```bash
. /__w/1/s/dotnet-arcade/eng/common/renovate.env
renovate 2>&1 | tee /tmp/renovate.out
renovateExit=${PIPESTATUS[0]}
if grep -q '^ WARN:' /tmp/renovate.out; then
echo "##vso[task.logissue type=warning]Renovate produced warnings."
echo "##vso[task.complete result=SucceededWithIssues]"
fi
exit $renovateExit
```

The `renovate 2>&1 | tee ...` pipeline may keep the bash process alive if the Renovate container's child processes (spawned via `postUpgradeTasks`) don't fully terminate or close their inherited file descriptors. The `unhandledRejection` from the missing `pwsh` binary suggests the spawned process errored out but may not have been cleaned up cleanly, leaving `tee` waiting on an open pipe.

### Underlying Renovate error (for context)

The `postUpgradeTasks` tried to run `pwsh` which is not installed in the `azurelinux-3.0-renovate-43-amd64` container image:

```
ERROR: unhandledRejection
"code": "ENOENT",
"syscall": "spawn pwsh",
"path": "pwsh",
"spawnargs": ["./eng/dockerfile-templates/Get-GeneratedDockerfiles.ps1"],
"message": "spawn pwsh ENOENT"
```

The missing PowerShell is being addressed in https://github.com/dotnet/dotnet-buildtools-prereqs-docker/pull/1608. This issue is specifically about the pipeline hanging when `postUpgradeTasks` fail.

### Expected behavior

When Renovate exits (whether successfully or with an error), the bash wrapper script should complete promptly, not hang for 15 minutes.

### Environment

- Pipeline: `dotnet-docker-renovate`
- Build: [#2929718](https://dev.azure.com/dnceng/internal/_build/results?buildId=2929718)
- Container image: `mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-renovate-43-amd64`
- Renovate version: 43.77.3
- Template: `/eng/common/core-templates/stages/renovate.yml@arcade`

/cc @mthalman