nixosTests.virtualbox.net-hostonlyif: use dhcpcd

dhclient is no longer built by default in the dhcp package, so this
test has been broken since that change was made.  To fix, switch to
dhcpcd.  dhcpcd insists on writing into /var/run, so we need to ensure
that exists.

Fixes: a2c379d4b6 ("dhcp: make client and relay component optional")

nixos/tests/virtualbox.nix

@@ -17,7 +17,8 @@ let
       #!${pkgs.runtimeShell} -xe
       export PATH="${lib.makeBinPath [ pkgs.coreutils pkgs.util-linux ]}"
 
-      mkdir -p /run/dbus
+      mkdir -p /run/dbus /var
+      ln -s /run /var
       cat > /etc/passwd <<EOF
       root:x:0:0::/root:/bin/false
       messagebus:x:1:1::/run/dbus:/bin/false
@@ -309,10 +310,7 @@ let
   ];
 
   dhcpScript = pkgs: ''
-    ${pkgs.dhcp}/bin/dhclient \
-      -lf /run/dhcp.leases \
-      -pf /run/dhclient.pid \
-      -v eth0 eth1
+    ${pkgs.dhcpcd}/bin/dhcpcd eth0 eth1
 
     otherIP="$(${pkgs.netcat}/bin/nc -l 1234 || :)"
     ${pkgs.iputils}/bin/ping -I eth1 -c1 "$otherIP"