internal/config: don't read local override file
internal/config: don't read local override file

Fixes this error:

readOverrideFile(ctx, "local-override.yaml"): dialing: google: could not
find default credentials. See
https://developers.google.com/accounts/docs/application-default-credentials
for more information.

Change-Id: I9478707913487c753143df113ac4fba61bde8df3
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/329501
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite