ensure that GITHUB_STEP_SUMMARY is not set

Author: smdn

tests/Smdn.Extensions.Mtp.LiquidTestReports/Smdn.Extensions.Mtp.LiquidTestReports/LiquidTestReportsConverter.cs

@@ -486,19 +486,20 @@ GitHub Actions step summary line #3
     var stepSummaryFile = isEnvVarSet
       ? new FileInfo(GetFilePathForTestCase($"{nameof(GITHUB_STEP_SUMMARY)}-{enableOption}-{isEnvVarSet}", TemplateFileExtension))
       : null;
-    string? initialValueOfEnvVar = null;
+    var initialValueOfEnvVar = Environment.GetEnvironmentVariable(GITHUB_STEP_SUMMARY);
 
     try {
       if (isEnvVarSet) {
-        initialValueOfEnvVar = Environment.GetEnvironmentVariable(GITHUB_STEP_SUMMARY);
-
         Environment.SetEnvironmentVariable(GITHUB_STEP_SUMMARY, stepSummaryFile!.FullName);
 
         await File.WriteAllTextAsync(
           path: stepSummaryFile.FullName,
           contents: InitialStepSummaryFileContents
         ).ConfigureAwait(false);
       }
+      else {
+        Environment.SetEnvironmentVariable(GITHUB_STEP_SUMMARY, null);
+      }
 
       await RunTestApplicationWithDefaultConfigurations(
         templateFileContents: TestCaseAndOutcomeTemplateFileContent,
@@ -548,8 +549,7 @@ await File.ReadAllTextAsync(stepSummaryFile!.FullName).ConfigureAwait(false),
       ).ConfigureAwait(false);
     }
     finally {
-      if (isEnvVarSet)
-        Environment.SetEnvironmentVariable(GITHUB_STEP_SUMMARY, initialValueOfEnvVar);
+      Environment.SetEnvironmentVariable(GITHUB_STEP_SUMMARY, initialValueOfEnvVar);
     }
   }
 }