[SourceBuild] Use --work-tree with git apply (#4254)

This makes things work bettern in a source-tarball build (where there
may not be a .git directory), or there might be a .git directory but
it's for a different repo than the one we are building.

eng/source-build/source-build.proj

@@ -55,7 +55,7 @@
     </ItemGroup>
 
     <Exec
-      Command="git apply --ignore-whitespace --whitespace=nowarn &quot;%(SourceBuildPatchFile.FullPath)&quot;"
+      Command="git --work-tree=$(ProjectRoot) apply --ignore-whitespace --whitespace=nowarn &quot;%(SourceBuildPatchFile.FullPath)&quot;"
       WorkingDirectory="$(ProjectRoot)"
       Condition="'@(SourceBuildPatchFile)' != ''" />
   </Target>