Add support for workspace git installation

- simplify the workaround conditions; paths will exist only on Windows
- reorder workaround conditions to prefer system `tar`

Author: dougbu

src/Framework/ref/Microsoft.AspNetCore.App.Ref.csproj

@@ -173,11 +173,12 @@ This package is an internal implementation of the .NET Core SDK and is not meant
     <!-- Work around issues finding the `tar` command on CI machines. -->
       <TarCommand>tar</TarCommand>
       <TarCommand
-          Condition="'$(OS)' == 'Windows_NT' AND Exists('$(SystemRoot)\System32\tar.exe')">"$(SystemRoot)\System32\tar.exe"</TarCommand>
+          Condition="Exists('$(MSBuildProgramFiles32)\Git\usr\bin\tar.exe')">"$(MSBuildProgramFiles32)\Git\usr\bin\tar.exe"</TarCommand>
       <TarCommand
-          Condition="'$(OS)' == 'Windows_NT' AND Exists('$(ProgramFiles)\Git\usr\bin\tar.exe')">"$(ProgramFiles)\Git\usr\bin\tar.exe"</TarCommand>
+          Condition="Exists('$(ProgramFiles)\Git\usr\bin\tar.exe')">"$(ProgramFiles)\Git\usr\bin\tar.exe"</TarCommand>
       <TarCommand
-          Condition="'$(OS)' == 'Windows_NT' AND Exists('$(MSBuildProgramFiles32)\Git\usr\bin\tar.exe')">"$(MSBuildProgramFiles32)\Git\usr\bin\tar.exe"</TarCommand>
+          Condition="Exists('$(AGENT_HOMEDIRECTORY)\externals\git\usr\bin\tar.exe')">"$(AGENT_HOMEDIRECTORY)\externals\git\usr\bin\tar.exe"</TarCommand>
+      <TarCommand Condition="Exists('$(SystemRoot)\System32\tar.exe')">"$(SystemRoot)\System32\tar.exe"</TarCommand>
 
       <!-- For the tar packed with git, transform e.g. "C:\root\AspNetCore\File.tar.gz" to "/C/root/AspNetCore/File.tar.gz". -->
       <_TarArchiveOutputPath>$(TarArchiveOutputPath)</_TarArchiveOutputPath>