cmd/gitmirror: update build image to Go 1.14
cmd/gitmirror: update build image to Go 1.14

The timeout added in CL 35124 doesn't seem to be working as intended.
Update to a supported version of Go before investigating further.

Also remove the explicit GOPROXY override, since the default behavior
as of Go 1.13 uses the Go module mirror¹.

¹ https://golang.org/doc/go1.13#introduction

For golang/go#35124.

Change-Id: Ia01abc530f0284791fe6e57ce5700c0ebc75268e
Reviewed-on: https://go-review.googlesource.com/c/build/+/227858
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

golang/build,golang/build,golang/build,golang/build