internal/config: add OnGCP
internal/config: add OnGCP

Add a method that reports whether the binary is on GCP.  We use the
GAE_ENV env var to detect AppEngine, and an env var we'll set in the
k8s deployment manifest to detect GKE.

Change-Id: Ieb026461d978d3fa9e81dd4d84ed6af085ec3a1f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/251058
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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