Gate GHA xunit config on GITHUB_ACTIONS environment variable

Ensures the no-parallelization config is only used when actually running on
GitHub Actions, not when DisableTestParallelization is passed locally.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: radical

tests/Directory.Build.targets

@@ -8,7 +8,7 @@
     <XunitRunnerJson Condition="'$(XunitRunnerJson)' == '' and '$(PrepareForHelix)' == 'true'">$(RepoRoot)tests\helix\xunit.runner.json</XunitRunnerJson>
     <!-- On resource-constrained CI runners (e.g. 2-core Windows), disable parallel test runs
          to prevent concurrent DCP processes from exhausting CPU. See #15832. -->
-    <XunitRunnerJson Condition="'$(XunitRunnerJson)' == '' and '$(DisableTestParallelization)' == 'true'">$(RepoRoot)tests\github-actions\xunit.runner.json</XunitRunnerJson>
+    <XunitRunnerJson Condition="'$(XunitRunnerJson)' == '' and '$(DisableTestParallelization)' == 'true' and '$(ContinuousIntegrationBuild)' == 'true'">$(RepoRoot)tests\github-actions\xunit.runner.json</XunitRunnerJson>
     <XunitRunnerJson Condition="'$(XunitRunnerJson)' == ''">$(RepositoryEngineeringDir)testing\xunit.runner.json</XunitRunnerJson>
 
     <!-- Properties to allow control tests to run, useful for local command line runs -->