Currently Kokoro Win2016 image has two discs; the main one (220GB, ~15% free) and the additional one (100GB, ~99% free). Recently Bazel RBE started failing with Not enough space because Bazel uses the main drive which doesn't have much free space for output. From the previous trial, ~30GB is needed for the bazel output when running all tests under //tests/.. So let's change the Bazel output directory to use the additional one (mounted to T:) which has more free space.

Space usage (for the reference)

kbuilder@kokoro-winserver2016 /tmpfs/_bazel_output/execroot/com_github_grpc_grpc/bazel-out/x64_windows-fastbuild
$ du --max-depth 2 -h | sort -h
301K	./bin/third_party
73M	./testlogs
73M	./testlogs/test
343M	./bin/src
734M	./bin/_objs
1.3G	./bin/external
24G	./bin/test
27G	.
27G	./bin