cmd/go: skip TestScript/mod_replace_gopkgin

(Until it can be made hermetic.)

The gopkg.in service has had a lot of flakiness lately. Go users in
general are isolated from that flakiness by the Go module mirror
(proxy.golang.org), but this test intentionally bypasses the module
mirror because the mirror itself uses cmd/go to download the module.

In the long term, we can redirect the gopkg.in URL to the local
(in-process) vcweb server added for #27494.

In the meantime, let's skip the test to reduce the impact of upstream
outages.

For #54503.

Change-Id: Icf3de7ca416db548e53864a71776fe22b444fcea
Reviewed-on: https://go-review.googlesource.com/c/go/+/454503
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/go/testdata/script/mod_replace_gopkgin.txt

@@ -4,6 +4,9 @@
 # even if there is an explicit go.mod file containing the
 # gopkg.in path.
 
+skip 'skipping test that depends on an unreliable third-party server; see https://go.dev/issue/54503'
+	# TODO(#54043): Make this test hermetic and re-enable it.
+
 [short] skip
 [!net] skip
 [!git] skip