test: fix prerelease tests (#239)

The heads of google-cloud-bigquery and google-cloud-bigquery-storage packages are installed with -e. This is leading to `from google.cloud import bigquery` looking into the folder of the latter and running into "ImportError: cannot import name 'bigquery' from 'google.cloud' (.../google-cloud-bigquery-storage/google/cloud/__init__.py). Removing -e from google-cloud-bigquery installation gets rid of this error.

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Fixes internal issue 313701211 🦕

noxfile.py

@@ -548,7 +548,6 @@ def prerelease(session: nox.sessions.Session, tests_path):
     # Ensure we catch breaking changes in the client libraries early.
     session.install(
         "--upgrade",
-        "-e",
         "git+https://github.com/googleapis/python-bigquery.git#egg=google-cloud-bigquery",
     )
     already_installed.add("google-cloud-bigquery")