ci: use python3 instead of python3.6 in build.sh (googleapis#425)

* ci: skip docfx in main 'Kokoro' presubmit

* fix: specify default sessions in noxfile

* fix: use python3 instead of 3.6

* fix: add NOX_SESSION to pass down envvars

* fix: remove quotes arround sessions

Co-authored-by: Tim Swast <swast@google.com>

.kokoro/build.sh

@@ -34,16 +34,16 @@ export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/service-account.json
 export PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/project-id.json")
 
 # Remove old nox
-python3.6 -m pip uninstall --yes --quiet nox-automation
+python3 -m pip uninstall --yes --quiet nox-automation
 
 # Install nox
-python3.6 -m pip install --upgrade --quiet nox
-python3.6 -m nox --version
+python3 -m pip install --upgrade --quiet nox
+python3 -m nox --version
 
 # If NOX_SESSION is set, it only runs the specified session,
 # otherwise run all the sessions.
 if [[ -n "${NOX_SESSION:-}" ]]; then
-    python3.6 -m nox -s "${NOX_SESSION:-}"
+    python3 -m nox -s ${NOX_SESSION:-}
 else
-    python3.6 -m nox
+    python3 -m nox
 fi

.trampolinerc

@@ -18,12 +18,14 @@
 required_envvars+=(
     "STAGING_BUCKET"
     "V2_STAGING_BUCKET"
+    "NOX_SESSION"
 )
 
 # Add env vars which are passed down into the container here.
 pass_down_envvars+=(
     "STAGING_BUCKET"
     "V2_STAGING_BUCKET"
+    "NOX_SESSION"
 )
 
 # Prevent unintentional override on the default image.