Update:

Setting global environment variables should still be performed in /etc/environment, but as you pointed out, using sudo -s has the effect of these variables are "vanished".

The reason for that is sudo has a policy of resetting the Environment and setting a secure path. It is enabled by default:

/etc/sudoers:



Defaults  env_reset

Defaults  secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

Thus whatever is set in the /etc/environment for the path is overridden by sudo.

The manual page for sudoers states:

   env_reset       If set, sudo will reset the environment to only contain

                   the LOGNAME, MAIL, SHELL, USER, USERNAME and the SUDO_*

                   variables.  Any variables in the caller's environment

                   that match the env_keep and env_check lists are then

                   added.  The default contents of the env_keep and

                   env_check lists are displayed when sudo is run by root

                   with the -V option.  If the secure_path option is set,

                   its value will be used for the PATH environment

                   variable.  This flag is on by default.

As a workaround, you can use sudo su that will provide a shell with root privileges but containing the right PATH.

Original Answer

You should set it in /etc/environment.

Try sudo YOUR_TEXT_EDITOR /etc/environment (make sure to create a backup first).

For more information: EnvironmentVariables

System-wide environment variables

Environment variable settings that affect the system as a whole
(rather then just a particular user) should not be placed in any of
the many system-level scripts that get executed when the system or the
desktop session are loaded, but into

/etc/environment - This file is specifically meant for system-wide environment variable settings. It is not a script file, but rather
consists of assignment expressions, one per line. Specifically, this
file stores the system-wide locale and path settings.

Not recommended:

/etc/profile - This file gets executed whenever a bash login shell is entered (e.g. when logging in from the console or over ssh), as
well as by the DisplayManager when the desktop session loads. This is
probably the file you will get referred to when asking veteran UNIX
system administrators about environment variables. In Ubuntu, however,
this file does little more then invoke the /etc/bash.bashrc file.

/etc/bash.bashrc - This is the system-wide version of the ~/.bashrc file. Ubuntu is configured by default to execute this file
whenever a user enters a shell or the desktop environment.