Don't overwrite cache timestamp until after build has been compiled and saved.

I am using asset compress build_ini task on our live server in our deploy script, but was having problems because the build timestamp cache file was being overwritten before the compiled file was generated, which caused visitors to get 404 errors when visiting the site, which were then cached on our CDN causing problems for everyone.
Yep, this is what was happening before, and is why in the build process the temporary cache name is used. You might have missed the part where I changed `$this->Cacher->setTimestamp($build, 0);` to be `$this->Cacher->invalidate($build);`. By using invalidate() I am telling AssetCache to use the temporary name during the build process, so that it doesn't affect the site.