[20.10 backport] Update GOPROXY to use default with fallback

[thaJeztah]

• backport of [master] Update GOPROXY to use default with fallback #745

Use the default proxy, to assist with LTS distros that use old git versions
but fallback on any error (instead of only on 404 and 410).

From the Go documentation; https://go.dev/ref/mod#goproxy-protocol

List elements may be separated by commas (,) or pipes (|), which determine error
fallback behavior. When a URL is followed by a comma, the go command falls back
to later sources only after a 404 (Not Found) or 410 (Gone) response. When a URL
is followed by a pipe, the go command falls back to later sources after any error,
including non-HTTP errors such as timeouts. This error handling behavior lets a
proxy act as a gatekeeper for unknown modules. For example, a proxy could respond
with error 403 (Forbidden) for modules not on an approved list (see Private proxy
serving private modules).

(cherry picked from commit 72d51db)

[thaJeztah]

all green 🎉