docs: enable pipstar for doc building (#3427)

Since we want to enable pipstar by default, enable it for our docs
builds as a way to
try it out consistently.

docs/readthedocs_build.sh

diff --git a/docs/readthedocs_build.sh b/docs/readthedocs_build.sh
index ec5390b..06ac769 100755
--- a/docs/readthedocs_build.sh
+++ b/docs/readthedocs_build.sh
@@ -12,7 +12,10 @@
 # In order to get the build number, we extract it from the host name
 extra_env+=("--//sphinxdocs:extra_env=HOSTNAME=$HOSTNAME")
 
+export RULES_PYTHON_ENABLE_PIPSTAR=1
+
 set -x
+export RULES_PYTHON_ENABLE_PIPSTAR=1
 bazel run \
   --config=rtd \
   "--//sphinxdocs:extra_defines=version=$READTHEDOCS_VERSION" \