Inject DOTNET_CLI_USE_MSBUILD_SERVER env var for apphost builds and runs, using configuration for value

[mitchdenny]

This issue tracks the enhancement to the Aspire CLI to always set the DOTNET_CLI_USE_MSBUILD_SERVER environment variable for all dotnet build and dotnet run (when noBuild == false) calls for apphost projects. The key technical implementation details are as follows:

Technical Implementation Details

1. Always inject the DOTNET_CLI_USE_MSBUILD_SERVER environment variable into the environment dictionary passed to the build/run process for apphost builds.
2. The value should be sourced from configuration["DOTNET_CLI_USE_MSBUILD_SERVER"]. This allows test scenarios to override the value using configuration.
3. If configuration["DOTNET_CLI_USE_MSBUILD_SERVER"] is not set, default the value to "true".
4. Do not check for or rely on any preexisting value in the env dictionary passed to the runner; always explicitly set the variable.
5. In DotNetCliRunner:
6. In BuildAsync(...): Always inject the variable before calling ExecuteAsync.
7. In RunAsync(...): Inject the variable only if noBuild == false, before calling ExecuteAsync.
8. Ensure that the DotNetCliRunner has access to an IConfiguration instance, either via dependency injection or other means.
9. Update or add tests to ensure:

• The variable defaults to "true" when not set in configuration.
• The variable can be overridden via configuration for testing or explicit opt-out.
• The environment variable is always present in the effective environment used for the build/run process.