Avoid installing NUnit if its already installed. (#1500)

build/InstallNugets.sh

@@ -1,3 +1,9 @@
-curl -O -L https://github.com/nunit/nunit-console/releases/download/v3.9/NUnit.Console-3.9.0.zip
 BUILD_DIR=$(dirname -- $0)
-unzip NUnit.Console-3.9.0.zip -d $BUILD_DIR/../deps/NUnit.Console-3.9.0
+NUNIT=NUnit.Console-3.9.0
+
+if [ -e $BUILD_DIR/../deps/$NUNIT/nunit3-console.exe ]; then
+  exit 0
+fi
+
+curl -O -L https://github.com/nunit/nunit-console/releases/download/v3.9/$NUNIT.zip
+unzip $NUNIT.zip -d $BUILD_DIR/../deps/$NUNIT