[Flaky Test] Microsoft.Build.BuildCheck.UnitTests.EndToEndTests.TestBuildCheckTemplate

**Flaky-test key** (automated de-duplication — do not edit):
```text
flaky-test-id: Microsoft.Build.BuildCheck.UnitTests.EndToEndTests.TestBuildCheckTemplate
```

## Evidence

- **Distinct sources:** 6 (2 approved PRs + 4 rolling `main` builds)
- **Affected PRs:** #14318, #14371
- **Rolling build IDs:** [1498681](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1498681), [1501229](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1501229), [1507966](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1507966), [1509637](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1509637)
- **Total failures:** 6
- **Legs:** CoreOnMac, CoreOnWindows
- **TFMs:** net10.0
- **Assembly:** Microsoft.Build.BuildCheck.UnitTests
- **First seen:** 2026-07-08 | **Last seen:** 2026-07-15
- **Distinct error hashes:** 6 (sporadic, non-deterministic failure signatures)
- **Sample build:** [1506605](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1506605)

## Sample error

```
System.TimeoutException : Test failed due to timeout: process 70200 is active for more than 30 sec.
```

## Fully-qualified test name

`Microsoft.Build.BuildCheck.UnitTests.EndToEndTests.TestBuildCheckTemplate`
Assembly: `Microsoft.Build.BuildCheck.UnitTests`
Project: `src/BuildCheck.UnitTests/`

## Action

This test will be **quarantined** via `[ActiveIssue("https://github.com/dotnet/msbuild/issues/")]` (from `Microsoft.DotNet.XUnitV3Extensions`, namespace `Xunit`) until the underlying nondeterminism is fixed. The quarantine pipeline (def 344) will continue collecting signal over time.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [Flaky Test Triage](https://github.com/dotnet/msbuild/actions/runs/29493043688) · 78.5 AIC · ⌖ 6.94 AIC · ⊞ 13.5K · [◷](https://github.com/search?q=repo%3Adotnet%2Fmsbuild+is%3Aissue+%22gh-aw-workflow-call-id%3A+dotnet%2Fmsbuild%2Fflaky-test-detector.agent%22&type=issues)

Start in src/BuildCheck.UnitTests/ and read Microsoft.Build.BuildCheck.UnitTests.EndToEndTests.TestBuildCheckTemplate. Run the test on the affected net10.0 CoreOnMac and CoreOnWindows legs, then inspect the reported 30-second process timeout and its differing failure signatures. Done means the test no longer shows the nondeterministic timeout and can leave quarantine.

Written by the indexing model from the issue text.