Inconsistent path lookup of iproute2 shell commands on Linux

[blenderfreaky]

Currently, iproute2 is called in two ways on linux:

• via routeCommand (on Linux that is set to = "/sbin/ip")
• and via path-lookup (shell.Exec(log, "ip", ...), in daemon/vpn/wireguard/wireguard_linux.go and daemon/v2r/v2ray_linux.go)

If ip is not in PATH but is present in /sbin/, this can cause confusing errors where it finds ip in some cases but not others.

This is a very niche issue, but we encountered this specific issue in the Nix packaging of ivpn, where we have to patch in a hardcoded path to the ip command (because standard FHS paths don't exist in Nix), while the PATH is kept very minimal. See here for more context: NixOS/nixpkgs#474377

I'd propose either changing all the path-lookups to use /sbin/ instead, or use routeCommand and introduce a second one for ip link (as used in wireguard_linux.go)