[Flaky Test] Quarantine 2 flaky tests (dotnet#14553)

## Summary

Quarantines 2 tests that have been repeatedly observed failing in CI
across independent rolling main builds.

---

### 1. `GetEnvVars` — Quarantine (6a)

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

- **Assembly**: `Microsoft.Build.Framework.UnitTests`
- **Distinct sources**: 3 rolling builds
- **Rolling build IDs**:
[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
- **First seen**: 2026-07-17 / **Last seen**: 2026-07-27

Tracked by dotnet#14541

---

### 2. `ServerSpawnAndReuseAreLoggedToBuildLog` — Quarantine (6a)

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

- **Assembly**: `Microsoft.Build.CommandLine.UnitTests`
- **Distinct sources**: 3 rolling builds
- **Rolling build IDs**:
[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**: FullOnWindows Release
- **Error hashes**: f2f3ee09, 4af42ebc, 347b6841 (3 distinct failure
signatures)
- **First seen**: 2026-07-17 / **Last seen**: 2026-07-27

Tracked by dotnet#14540




> [!WARNING]
> <details>
> <summary>Firewall blocked 2 domains</summary>
>
> The following domains were blocked by the firewall during workflow
execution:
>
> - `awmgmcpg`
> - `southcentralus0.in.applicationinsights.azure.com`
>> To allow these domains, add them to the `network.allowed` list in
your workflow frontmatter:
>
> ```yaml
> network:
>   allowed:
>     - defaults
>     - "awmgmcpg"
>     - "southcentralus0.in.applicationinsights.azure.com"
> ```
>
> See [Network
Configuration](https://github.github.com/gh-aw/reference/network/) for
more information.
>
> </details>


> Generated by [Flaky Test
Triage](https://github.com/dotnet/msbuild/actions/runs/30358735168) ·
sonnet46 · 119.8 AIC · ⌖ 6.34 AIC · ⊞ 14.7K ·
[◷](https://github.com/search?q=repo%3Adotnet%2Fmsbuild+%22gh-aw-workflow-id%3A+flaky-test-detector.agent%22&type=pullrequests)

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/Framework.UnitTests/CommunicationUtilities_Tests.cs

@@ -15,6 +15,7 @@ public class CommunicationUtilitiesTests
     /// <summary>
     /// Verify our custom way of getting env vars gives the same results as the BCL.
     /// </summary>
+    [ActiveIssue("https://github.com/dotnet/msbuild/issues/14541")]
     [Fact]
     public void GetEnvVars()
     {

src/MSBuild.UnitTests/MSBuildServer_Tests.cs

@@ -147,6 +147,7 @@ public void MSBuildServerTest()
             pidOfServerProcess.ShouldNotBe(newServerProcessId, "Node used by both the first and second build should not be the same.");
         }
 
+        [ActiveIssue("https://github.com/dotnet/msbuild/issues/14540")]
         [Fact]
         public void ServerSpawnAndReuseAreLoggedToBuildLog()
         {