internal/config: support env var for bucket
internal/config: support env var for bucket

The GCS bucket for overrides can now be configured with an environment
variable.

For golang/go#41483

Change-Id: Iba9f83afd7bc658d89d21f9d5ce693bbd142cb25
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/256017
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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