ref: use internal pypi for setup-python as well (#40334)

we've seen some significant flakiness with installing `pip` from public
pypi in GHA -- this should make it install from our pypi instead
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.github/actions/setup-sentry/action.yml

@@ -68,6 +68,7 @@ runs:
         fi
 
         echo "PIP_DISABLE_PIP_VERSION_CHECK=on" >> $GITHUB_ENV
+        echo "PIP_INDEX_URL=https://pypi.devinfra.sentry.io/simple" >> $GITHUB_ENV
         echo "PY_COLORS=1" >> "$GITHUB_ENV"
         echo "SENTRY_SKIP_BACKEND_VALIDATION=1" >> $GITHUB_ENV