Fix build and snk updates during template expansion

We inappropriately removed the required snk file for our own build in 4a0dface5e. This brings it back.
It also updates Expand-Template.ps1 to overwrite the existing one rather than creating a new one in an ignored location.

Expand-Template.ps1

@@ -101,8 +101,8 @@ try {
     git add "test/$LibraryName.Tests/$LibraryName.Tests.csproj"
 
     # Establish a new strong-name key
-    & $sn.Path -k 2048 src/strongname.snk
-    git add src/strongname.snk
+    & $sn.Path -k 2048 strongname.snk
+    git add strongname.snk
 
     # Replace placeholders in source files
     Replace-Placeholders -Path "src/$LibraryName/Calculator.cs" -Replacements @{