Selectively define VSInstallRoot

Commit 9b2d23b worked when using .NET Framework on Windows and
on macOS, but failed when using Windows + .NET Core, because
`dotnet build` *doesn't* define `%VSINSTALLROOT%` in that environment.

Update the `windows_dotnet_build` job so that `VSInstallRoot` is
defined.

Update the `NativeTiming` build so that it verifies that
`$(VSINSTALLROOT)` exists before trying to do anything with it.

build-tools/automation/azure-pipelines.yaml

@@ -53,7 +53,7 @@ jobs:
       msbuildArguments: /restore /t:Prepare
 
   - task: MSBuild@1
-    displayName: VSBuild Java.Interop.sln
+    displayName: MSBuild Java.Interop.sln
     inputs:
       solution: Java.Interop.sln
       configuration: $(Build.Configuration)
@@ -78,6 +78,9 @@ jobs:
   displayName: Windows - .NET Core
   pool: $(HostedWinVS2019)
   timeoutInMinutes: 20
+  variables:
+  - name: VSInstallRoot
+    value: C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise
   workspace:
     clean: all
   steps:

tests/NativeTiming/Directory.Build.targets

@@ -19,10 +19,13 @@
   </Target>
 
   <Target Name="_GetBuildComands">
-    <ItemGroup Condition=" '$(VSINSTALLROOT)' != '' ">
-      <_Vcvarsall Include="$(VSINSTALLROOT)\VC\Auxiliary\Build\vcvarsall.bat" />
+    <ItemGroup>
+      <_Vcvarsall
+          Condition=" '$(VSINSTALLROOT)' != '' And Exists('$(VSINSTALLROOT)') "
+          Include="$(VSINSTALLROOT)\VC\Auxiliary\Build\vcvarsall.bat"
+      />
     </ItemGroup>
-    <PropertyGroup Condition=" '$(VSINSTALLROOT)' != '' And '$(_Vcvarsall->Count())' != '0' ">
+    <PropertyGroup Condition=" '@(_Vcvarsall->Count())' != '0' ">
       <_Vcvarsall>%(_Vcvarsall.Identity)</_Vcvarsall>
       <_PrepareToolchain>call "$(_Vcvarsall)" x86_amd64 &amp;&amp; </_PrepareToolchain>
     </PropertyGroup>