fix: Flaky CI tests by increase build timeout

[filzrev]

This PR intended to fix #2853

What's changed in this PR
Increase BuildTimeout value to value to 240 seconds (Default: 120 seconds)
of following tests.

• WasmIsSupported
• UserCanSpecifyCustomBuildConfiguration

Note:
Following FactEnvSpecific is left as is.

BenchmarkDotNet/tests/BenchmarkDotNet.IntegrationTests/CustomBuildConfigurationTests.cs

Line 21 in 6fcb40c

[FactEnvSpecific("Flaky, see https://github.com/dotnet/BenchmarkDotNet/issues/2376", EnvRequirement.NonFullFramework)]

Timeout error on NETFX might be resolved by this PR changes.
Though, it'll be handled by #2376

[filzrev]

macos (x64) CI failed on following error.

MemoryDiagnoserSupportsModernMono [FAIL]
2025-11-05T04:42:58.2674460Z [xUnit.net 00:35:03.19]       Assert.Equal() Failure: Values differ
2025-11-05T04:42:58.2682030Z [xUnit.net 00:35:03.19]       Expected: 40
2025-11-05T04:42:58.2705840Z [xUnit.net 00:35:03.19]       Actual:   null

Above error seems to be happen randomly. and cause flaky test.

And ubuntu-latest following MSB3277 warning happens.

2025-11-05T04:28:21.5570278Z /home/runner/work/BenchmarkDotNet/BenchmarkDotNet/.dotnet/sdk/8.0.410/Microsoft.Common.CurrentVersion.targets(2412,5): warning MSB3277: Found conflicts between different versions of "System.Runtime" that could not be resolved. [/home/runner/work/BenchmarkDotNet/BenchmarkDotNet/src/BenchmarkDotNet.Diagnostics.dotMemory/BenchmarkDotNet.Diagnostics.dotMemory.csproj::TargetFramework=net6.0]
2025-11-05T04:28:21.5579320Z /home/runner/work/BenchmarkDotNet/BenchmarkDotNet/.dotnet/sdk/8.0.410/Microsoft.Common.CurrentVersion.targets(2412,5): warning MSB3277: There was a conflict between "System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" and "System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a". [/home/runner/work/BenchmarkDotNet/BenchmarkDotNet/src/BenchmarkDotNet.Diagnostics.dotMemory/BenchmarkDotNet.Diagnostics.dotMemory.csproj::TargetFramework=net6.0]
2025-11-05T04:28:21.5587153Z /home/runner/work/BenchmarkDotNet/BenchmarkDotNet/.dotnet/sdk/8.0.410/Microsoft.Common.CurrentVersion.targets(2412,5): warning MSB3277:     "System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" was chosen because it was primary and "System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" was not. [/home/runner/work/BenchmarkDotNet/BenchmarkDotNet/src/BenchmarkDotNet.Diagnostics.dotMemory/BenchmarkDotNet.Diagnostics.dotMemory.csproj::TargetFramework=net6.0]
2025-11-05T04:28:21.5594052Z /home/runner/work/BenchmarkDotNet/BenchmarkDotNet/.dotnet/sdk/8.0.410/Microsoft.Common.CurrentVersion.targets(2412,5): warning MSB3277:     References which depend on "System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" [/home/runner/.nuget/packages/microsoft.netcore.app.ref/6.0.36/ref/net6.0/System.Runtime.dll]. [/home/runner/work/BenchmarkDotNet/BenchmarkDotNet/src/BenchmarkDotNet.Diagnostics.dotMemory/BenchmarkDotNet.Diagnostics.dotMemory.csproj::TargetFramework=net6.0]

I don't know why this warning occurred randomly.
Though it's caused by transitive package reference conflicts between .NET6 / .NET8
So add .NET8 TargetFrameworks to dotTrace/dotMemory project