ci: skip docfx in main 'Kokoro' presubmit (#423)

* ci: skip docfx in main 'Kokoro' presubmit

* fix: specify default sessions in noxfile

* add conditional for cd to project root

.kokoro/build.sh

@@ -15,7 +15,11 @@
 
 set -eo pipefail
 
-cd github/python-bigquery
+if [[ -z "${PROJECT_ROOT:-}" ]]; then
+    PROJECT_ROOT="github/python-bigquery"
+fi
+
+cd "${PROJECT_ROOT}"
 
 # Disable buffering, so that the logs stream through.
 export PYTHONUNBUFFERED=1

.kokoro/docs/docs-presubmit.cfg

@@ -15,3 +15,14 @@ env_vars: {
     key: "TRAMPOLINE_IMAGE_UPLOAD"
     value: "false"
 }
+
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/python-bigquery/.kokoro/build.sh"
+}
+
+# Only run this nox session.
+env_vars: {
+    key: "NOX_SESSION"
+    value: "docs docfx"
+}

noxfile.py

@@ -25,6 +25,18 @@
 BLACK_PATHS = ("docs", "google", "samples", "tests", "noxfile.py", "setup.py")
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
+# 'docfx' is excluded since it only needs to run in 'docs-presubmit'
+nox.options.sessions = [
+    "unit",
+    "system",
+    "snippets",
+    "cover",
+    "lint",
+    "lint_setup_py",
+    "blacken",
+    "docs",
+]
+
 
 def default(session):
     """Default unit test session.