Fix build_package_ruby.sh to pass shellcheck

tools/run_tests/artifacts/build_package_ruby.sh

@@ -15,7 +15,7 @@
 
 set -ex
 
-cd $(dirname $0)/../../..
+cd "$(dirname "$0")/../../.."
 
 base=$(pwd)
 
@@ -23,7 +23,7 @@ mkdir -p artifacts/
 
 # All the ruby packages have been built in the artifact phase already
 # and we only collect them here to deliver them to the distribtest phase.
-cp -r $EXTERNAL_GIT_ROOT/platform={windows,linux,macos}/artifacts/ruby_native_gem_*/* artifacts/ || true
+cp -r "$EXTERNAL_GIT_ROOT"/platform={windows,linux,macos}/artifacts/ruby_native_gem_*/* artifacts/ || true
 
 well_known_protos=( any api compiler/plugin descriptor duration empty field_mask source_context struct timestamp type wrappers )
 
@@ -43,16 +43,16 @@ for arch in {x86,x64}; do
   for plat in {windows,linux,macos}; do
     input_dir="$EXTERNAL_GIT_ROOT/platform=${plat}/artifacts/protoc_${plat}_${arch}"
     output_dir="$base/src/ruby/tools/bin/${ruby_arch}-${plat}"
-    mkdir -p $output_dir/google/protobuf
-    mkdir -p $output_dir/google/protobuf/compiler  # needed for plugin.proto
-    cp $input_dir/protoc* $output_dir/
-    cp $input_dir/grpc_ruby_plugin* $output_dir/
+    mkdir -p "$output_dir"/google/protobuf
+    mkdir -p "$output_dir"/google/protobuf/compiler  # needed for plugin.proto
+    cp "$input_dir"/protoc* "$output_dir"/
+    cp "$input_dir"/grpc_ruby_plugin* "$output_dir"/
     for proto in "${well_known_protos[@]}"; do
-      cp $base/third_party/protobuf/src/google/protobuf/$proto.proto $output_dir/google/protobuf/$proto.proto
+      cp "$base/third_party/protobuf/src/google/protobuf/$proto.proto" "$output_dir/google/protobuf/$proto.proto"
     done
   done
 done
 
-cd $base/src/ruby/tools
+cd "$base/src/ruby/tools"
 gem build grpc-tools.gemspec
-cp ./grpc-tools*.gem $base/artifacts/
+cp ./grpc-tools*.gem "$base/artifacts/"