cmd/go: remove test dependency on gopkg.in service [1.18 backport]

[gopherbot]

@bcmills requested issue #54503 to be considered for backport to the next 1.18 minor release.

@gopherbot, please backport to Go 1.18 and 1.19. This test is very flaky lately, so we should at least backport a skip for it.

[gopherbot]

Change https://go.dev/cl/454840 mentions this issue: [release-branch.go1.18] cmd/go: skip TestScript/mod_replace_gopkgin

[gopherbot]

Closed by merging 0b12684 to release-branch.go1.18.