reversesshfs: `ReverseSSHFS.Start()` does not capture stderr on executing command

[norio-nomura]

This interferes with parsing lima's ha.stderr.log with jq --slurp.
e.g.

$ limactl start template:debian --vm-type qemu --tty=false --mount-only ~/Library --mount-only ~/Library/Caches:w --log-level=error
$ jq --slurp '.[]|select(.level == "error" or .error)' ~/.lima/debian/ha.stderr.log
jq: parse error: Invalid literal at line 59, column 12
$ sed -n '58,60p' ~/.lima/debian/ha.stderr.log
{"level":"debug","msg":"executing OpenSSH SFTP Server: /usr/libexec/sftp-server [/usr/libexec/sftp-server -e -d /Users/norio/Library/Caches]","time":"2025-10-08T13:58:09+09:00"}
fusermount3: user has no write access to mountpoint /Users/norio/Library/Caches
{"level":"debug","msg":"waiting for remote ready","time":"2025-10-08T13:58:09+09:00"}
$ limactl --version
limactl version 2.0.0-alpha.2-50-g89c500c6.m