docs: filter env output in Read the Docs build (#4450)

Logging all environment variables during Read the Docs builds dumps
noisy and potentially sensitive unrelated variables into build logs.

Filter `env` output to `READTHEDOC*` variables so logs retain the
Read the Docs context needed for debugging builds.

.readthedocs.yml

diff --git a/.readthedocs.yml b/.readthedocs.yml
index 6613d49..6e28fa3 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -6,7 +6,9 @@
   tools:
     nodejs: "19"
   commands:
-    - env
+    # Log RTD env vars to help debug builds without dumping noisy or sensitive
+    # unrelated env vars.
+    - env | grep '^READTHEDOC*'
     - npm install -g @bazel/bazelisk
     - bazel version
     # Put the actual build behind a shell script because its easier to modify than