Get rid of SSL_CERT_FILE env entirely

src/python/grpcio/tests/interop/client.py

src/python/grpcio/tests/interop/resources.py

src/python/grpcio/tests/unit/resources.py

src/ruby/bin/apis/pubsub_demo.rb

src/ruby/ext/grpc/rb_channel_credentials.c

src/ruby/pb/test/client.rb

tools/run_tests/run_interop_tests.py