Save the file to your scripts/vscripts folder as whichskybox.nut
Add a logic_script entity to your map, turn off smartedit and add a "vscripts" key with a value of whichskybox.nut
Obviously you would have to adjust fog and lighting entities as well for it to not look terrible.
Skyboxes <- [
"sky_l4d_c1_1_hdr",
"sky_l4d_c1_2_hdr",
"sky_l4d_c2m1_hdr",
"sky_l4d_night02_hdr",
"sky_l4d_predawn02_hdr",
"sky_l4d_c4m1_hdr",
"sky_l4d_c4m4_hdr",
"sky_l4d_c5_1_hdr",
"sky_l4d_c6m1_hdr"
]
worldspawn <- Entities.FindByClassname (null, "worldspawn");
local i = RandomInt(0,Skyboxes.len()-1);
printl("Skybox is "+Skyboxes[i]);
printl( worldspawn.__KeyValueFromString("skyname",Skyboxes[i]) );