Switching to linux for proto gen, fix output paths (#4831)

* try switching to linux for proto gen

* update platform string

.yamato/protobuf-generation-test.yml

-test_mac_protobuf_generation:
+test_linux_protobuf_generation:
-    type: Unity::VM::osx
-    image: package-ci/mac:stable
-    flavor: b1.small
+    type: Unity::VM
+    image: package-ci/ubuntu:stable
+    flavor: b1.large
-    HOMEBREW_NO_AUTO_UPDATE: "1"
-      brew install nuget
+      sudo apt-get update && sudo apt-get install -y python3-venv nuget
+      python3 -m venv venv && source venv/bin/activate
-      python3 -m venv venv
-      . venv/bin/activate
-      cd protobuf-definitions
-      chmod +x Grpc.Tools.$GRPC_VERSION/tools/macosx_x64/protoc
-      chmod +x Grpc.Tools.$GRPC_VERSION/tools/macosx_x64/grpc_csharp_plugin
-      COMPILER=Grpc.Tools.$GRPC_VERSION/tools/macosx_x64 ./make.sh
+      pushd protobuf-definitions
+      chmod +x Grpc.Tools.$GRPC_VERSION/tools/linux_x64/protoc
+      chmod +x Grpc.Tools.$GRPC_VERSION/tools/linux_x64/grpc_csharp_plugin
+      COMPILER=Grpc.Tools.$GRPC_VERSION/tools/linux_x64 ./make.sh
+      popd
       mkdir -p artifacts
       touch artifacts/proto.patch
       git diff --exit-code -- :/ ":(exclude,top)$CS_PROTO_PATH/*.meta"                          \