chore: allow bigframes tests to use compute engine credentials (#17824)

This is in response to internal issue b/537733439 where the tests are
being skipped on the internal test cluster
🦕

Author: tswast

packages/bigframes/noxfile.py

@@ -334,9 +334,6 @@ def run_system(
         CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
     )
 
-    if "GOOGLE_APPLICATION_CREDENTIALS" not in os.environ:
-        session.skip("Credentials must be set via environment variable")
-
     # Check the value of `RUN_SYSTEM_TESTS` env var. It defaults to true.
     if os.environ.get("RUN_SYSTEM_TESTS", "true") == "false":
         session.skip("RUN_SYSTEM_TESTS is set to false, skipping")
@@ -595,9 +592,6 @@ def docfx(session):
 
 
 def prerelease(session: nox.sessions.Session, tests_path, extra_pytest_options=()):
-    if "GOOGLE_APPLICATION_CREDENTIALS" not in os.environ:
-        session.skip("Credentials must be set via environment variable")
-
     constraints_path = str(
         CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
     )