cmd/go: skip mod_convert_tsv_insecure test when no network is avaliable

Fixes #35703

Change-Id: I476efad38897cae93f298af86784bbc2cc2449a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/208037
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

 env GO111MODULE=on
 
+[!net] skip
+[!exec:git] skip
+
 # secure fetch should report insecure warning
 cd $WORK/test
 go mod init