fix: Fix script with condition for Windows instead of defaulting to it

Author: purvavasudeo

generateprotos.sh

@@ -8,9 +8,11 @@
 # - GAPIC ShowCase
 
 # Cross-platform tools
-protoc_executable="tools/protoc.exe" # default to Windows
-grpc_csharp_executable="tools/grpc_csharp_plugin.exe" # default to Windows
 case "$OSTYPE" in
+  win* | msys* | cygwin*)
+    protoc_executable="tools/protoc.exe" # default to Windows
+    grpc_csharp_executable="tools/grpc_csharp_plugin.exe" # default to Windows
+    ;; 
   linux*)
     protoc_executable="tools/protoc"
     grpc_csharp_executable="tools/grpc_csharp_plugin"