[Flaky Test] Microsoft.Build.UnitTests.TaskEnvironment_Tests.TaskEnvironment_SetEnvironment_ShouldReplaceAllVariables

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

## Evidence

Repeatedly failing in test method cleanup (observed as "Test Method Cleanup Failure" in TRX logs) for both `environmentType: "Stub"` and `environmentType: "Multithreaded"` variants.

- **Distinct sources:** 3 (0 PRs + 3 rolling builds) per variant
- **Rolling builds:** [1514373](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1514373), [1517682](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1517682), [1527742](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1527742)
- **Legs:** CoreOnLinux
- **TFMs:** net10.0, net11.0
- **Assembly:** Microsoft.Build.Engine.UnitTests
- **First seen:** 2026-07-17 | **Last seen:** 2026-07-27

The failures appear in cleanup (after the test body completes), suggesting a resource leak or environment variable cleanup issue.

## Fully-qualified test name

`Microsoft.Build.UnitTests.TaskEnvironment_Tests.TaskEnvironment_SetEnvironment_ShouldReplaceAllVariables`

Source: `src/Build.UnitTests/BackEnd/TaskEnvironment_Tests.cs`

## Next steps

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.

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> Generated by [Flaky Test Triage](https://github.com/dotnet/msbuild/actions/runs/30451703648) · sonnet46 · 150.7 AIC · ⌖ 5.46 AIC · ⊞ 14.7K · [◷](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)

Read src/Build.UnitTests/BackEnd/TaskEnvironment_Tests.cs, focusing on TaskEnvironment_SetEnvironment_ShouldReplaceAllVariables and its cleanup behavior. Run the test for both Stub and Multithreaded variants on CoreOnLinux and compare the cleanup failures. Done means repeated runs complete without cleanup failures and the quarantine is no longer needed.

Written by the indexing model from the issue text.