cmd/go: avoid password prompts in TestScript/mod_get_private_vcs
cmd/go: avoid password prompts in TestScript/mod_get_private_vcs

In some cases, this test would prompt for interactive SSH passwords in
order to authenticate to github.com over SSH. Setting GIT_SSH_COMMAND
to /bin/false prevents that, while still provoking the desired Git
failure mode.

Updates #44904.

Change-Id: Idc9fe9f47d2ccb6c8a4ea988b73d9c8c774e4079
Reviewed-on: https://go-review.googlesource.com/c/go/+/300156
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Kevin Burke <685caccd7011cfe58b8f9ef79ff9dca388f03c79@inburke.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Kevin Burke <685caccd7011cfe58b8f9ef79ff9dca388f03c79@inburke.com>

nawawi/go,nawawi/go,cloudflare/go,golang/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,golang/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go