Fix typo correctly (#40727)

.azure/pipelines/ci.yml

@@ -295,7 +295,7 @@ stages:
           inputs:
             packageType: sdk
             version: 3.1.x
-            installationPath: $(Build.SourcesDirectory)./dotnet
+            installationPath: $(Build.SourcesDirectory)/.dotnet
             workingDirectory: $(Build.SourcesDirectory)
         - powershell: . $(Build.SourcesDirectory)/activate.ps1;
             dotnet tool install BinLogToSln