x/build/cmd/relui,x/build/cmd/release: convert to gRPC coordinator API

[heschi]

The HTTP based API is obsolete. Port relui over.

cc @golang/release

[gopherbot]

Change https://go.dev/cl/422096 mentions this issue: cmd/relui: use gRPC buildlet

[gopherbot]

Change https://go.dev/cl/422095 mentions this issue: buildlet: create gRPC client

[gopherbot]

Change https://go.dev/cl/422636 mentions this issue: internal/iapclient: check OnGCE before reading project ID

[heschi]

Also need to do cmd/release.

[gopherbot]

Change https://go.dev/cl/424000 mentions this issue: cmd/release: use gRPC to coordinator