cmd/go: avoid ssh known_hosts prompts on test

TestScripts/mod_get_svn would stop with the following prompt if the real
user didn't have vcs-test.golang.org in their known_hosts file:

	The authenticity of host 'vcs-test.golang.org (35.184.38.56)' can't be established.
	ECDSA key fingerprint is SHA256:[...]
	Are you sure you want to continue connecting (yes/no/[fingerprint])?

This was bad because it relied on the user's real ssh known_hosts file.
Worse even, if the user didn't expert or notice the prompt, it could
hang a 'go test' run for quite a while.

Work around that by forcing svn to not use ssh at all. Other potentially
better approaches were tried, but none worked on svn 1.12.2 with openssh
8.0p1.

Fixes #33883.

Change-Id: I2f925fa892f2fa53c77d86b0034141162517ee69
Reviewed-on: https://go-review.googlesource.com/c/go/+/199142


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>