fix(s3-deployment): bundle modules correctly (#1154)

--prefix never install modules which install into system python and
these modules won't include in bundle.zip, so ignore whether or not
modules already installed.

packages/@aws-cdk/aws-s3-deployment/lambda/build.sh

@@ -27,7 +27,7 @@ cd ${staging}
 # install python requirements
 # Must use --prefix to because --target cannot be used on
 # platforms that have a default --prefix set.
-pip3 install --prefix ${piptemp} -r ${staging}/requirements.txt
+pip3 install --ignore-installed --prefix ${piptemp} -r ${staging}/requirements.txt
 mv ${piptemp}/lib/python*/site-packages/* .
 
 # create archive