[CI] Fix azdo builds for main

[radical]

What broke?

Template tests run dotnet test --list-tests to get the list of tests so they can
be run on separate helix jobs. This run of dotnet failed the builds on
azdo with:

  You must install or update .NET to run this application.

  App: /mnt/vss/_work/1/s/artifacts/bin/Aspire.Templates.Tests/Release/net8.0/Aspire.Templates.Tests
  Architecture: x64
  Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)
  .NET location: /usr/lib/dotnet

  The following frameworks were found:
    6.0.36 at [/usr/lib/dotnet/shared/Microsoft.NETCore.App]

  Learn about framework resolution:
  https://aka.ms/dotnet/app-launch-failed

  To install missing framework, download:
  https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=8.0.0&arch=x64&rid=ubuntu.22.04-x64
/mnt/vss/_work/1/s/tests/Directory.Build.targets(39,5): error MSB3073: The command ""/mnt/vss/_work/1/s/artifacts/bin/Aspire.Templates.Tests/Release/net8.0/Aspire.Templates.Tests" --filter-no>
 ##[error]tests/Directory.Build.targets(39,5): error MSB3073: (NETCORE_ENGINEERING_TELEMETRY=Build) The command ""/mnt/vss/_work/1/s/artifacts/bin/Aspire.Templates.Tests/Release/net8.0/Aspire>

Why did it break?

[1] added a change where that dotnet test was run with
EnvironmentVariables="DOTNET_ROOT=$(DotNetRoot);DOTNET_ROOT_X86=$(DotNetRoot)x86",
so it does not use the system dotnet. This helped on Azdo builds where
we have older system dotnet (6.0x).

Then [2] removed that change so the system dotnet on github actions
could be used because a newer one was being installed. But this now
broke the Azdo builds.

To fix this set the DOTNET_ROOT on Azdo builds, so the correct dotnet is
used.

References:
1.
commit 0af9e78
Author: Youssef Victor youssefvictor00@gmail.com
Date: Sat Apr 19 14:34:54 2025 +0200

Revert to MTP and disable server capability as a workaround (#8833)

commit 6ca2de945a44868673e12fa43e2a76055c34bf4a
Author: Ankit Jain <radical@gmail.com>
Date:   Wed May 14 11:39:18 2025 -0400

    [CI] Add PR validation on macOS (#9287)

[radical]

These need to be validated before merging:

• Tests workflow on GH
• Azdo public pipeline
• Azdo internal pipeline

[Copilot]

Pull Request Overview

This PR fixes the Azure DevOps builds by ensuring that the correct .NET installation is used for the test and build steps.

• Updated the test step environment variable to set DOTNET_ROOT
• Added/updated the DOTNET_ROOT environment variable for the build step

Comments suppressed due to low confidence (1)

eng/pipelines/templates/BuildAndTest.yml:64

• [nitpick] The path separators for DOTNET_ROOT differ between the test step (using backslashes) and the build step (using forward slashes). Consider using consistent path separators to improve readability and reduce potential issues in cross-platform environments.

+        DOTNET_ROOT: $(Build.SourcesDirectory)\ .dotnet

[danmoseley]

lgtm if it works

[radical]

Azdo build - https://dev.azure.com/dnceng-public/public/_build/results?buildId=1042959&view=results

[radical]

azdo internal - https://dev.azure.com/dnceng/internal/_build/results?buildId=2710085&view=results

[RussKie]

Should #9261 be reinstated now?