SteamPipe update released - what does it mean for us?
Unfortunately valve broke the addons folder in portal 2 for some weird reason. Afaik they only changed the gcf's to vpk's. And moved the game from /username/ to /common/
I would really appreciate if anyone else could take a look at this.
(Partly) Good news! Good guy and the hero Josepezdj found this Steam discussion where you can find edited gameinfo.txt for some Portal mods. I tried to open their "Rexaura" gameinfo.txt edit and it looks like this:
"GameInfo"
{
game "Rexaura"
title "Rexaura"
type singleplayer_only
nodifficulty 1
hasportals 1
icon "resource/icon"
developer "Ben Bryant (Mevious)"
developer_url "http://www.moddb.com/mods/rexaura"
FileSystem
{
SteamAppId 400
SearchPaths
{
game+mod |gameinfo_path|.
platform |gameinfo_path|.
// We search VPK files before ordinary folders, because most files will be found in
// VPK and we can avoid making thousands of file system calls to attempt to open files
// in folders where they don't exist. (Searching a VPK is much faster than making an operating
// system call.)
game_lv portal/portal_lv.vpk
game+mod portal/portal_english.vpk
game+mod portal/portal_pak.vpk
game |all_source_engine_paths|hl2/hl2_textures.vpk
game |all_source_engine_paths|hl2/hl2_sound_vo_english.vpk
game |all_source_engine_paths|hl2/hl2_sound_misc.vpk
game |all_source_engine_paths|hl2/hl2_misc.vpk
platform |all_source_engine_paths|platform/platform_misc.vpk
// Now search loose files. We'll set the directory containing the gameinfo.txt file
// as the first "mod" search path (after any user customizations). This is also the one
// that's used when writing to the "mod" path.
mod+mod_write+default_write_path |gameinfo_path|.
// Add the mod directory as a game search path. This is also where where writes
// to the "game" path go.
game+game_write |gameinfo_path|.
// Where the game's binaries are
gamebin portal/bin
// Last, mount in shared Portal loose files
game |all_source_engine_paths|portal
// Last, mount in shared HL2 loose files
game |all_source_engine_paths|hl2
platform |all_source_engine_paths|platform
}
}
}
So what does this mean? Well it looks like we don't have to pack mods into .vpk, but we have to edit paths in gameinfo.txt.
You will have to add these two sections:
game_lv portal/portal_lv.vpk
game+mod portal/portal_english.vpk
game+mod portal/portal_pak.vpk
game |all_source_engine_paths|hl2/hl2_textures.vpk
game |all_source_engine_paths|hl2/hl2_sound_vo_english.vpk
game |all_source_engine_paths|hl2/hl2_sound_misc.vpk
game |all_source_engine_paths|hl2/hl2_misc.vpk
platform |all_source_engine_paths|platform/platform_misc.vpk
and
mod+mod_write+default_write_path |gameinfo_path|.
game+game_write |gameinfo_path|.
gamebin portal/bin
game |all_source_engine_paths|portal
game |all_source_engine_paths|hl2
platform |all_source_engine_paths|platform
(Both of these go into SearchPaths section).
Read the discussion for more info. Feel free to post here your discoveries!
Once we have updated the game, Portal, all wehave to do is to launch the hammer editor directly without the tools launcher. We've got all tools into the bin folder:
Steam/SteamApps/common/Portal/bin
So go there and find your hammer.exe file and just execute it
It's very cute now: it asks you what game are you going to map for 
(I haven't tried any compile or such... please try)
So, I bet this is somewhat what's happening here 
Can you confirm where is the map you are trying to build cubemaps for?
I'll do some tests later today in order to go deeper into this as well.
EDIT: Doing some googling it seems that all Steampipe source games (before Alien Swarm) fail to compile cubemaps
I'd suggest you to try first while running Portal, and for a copy of your map located into common/portal/maps to see if that makes any difference.