Setting system-wide environment variable using /etc/environment

The /etc/environment file is not a script file: AFAIK you can't use export there and it doesn't support variable expansion of the type $HOME, just simplevariable=value pairs. So to use that file, you'd need to simply append your path to the existing definition, like

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/home/username/R/bin"

However, setting the system-wide PATH to include a user's home directory in this way is a questionable practice IMHO: the normal way would be to use one of the user's own shell startup files: ~/.profile is usually the recommended one for environment variables, however it has the disadvantage of only being invoked by login shells, so in order to get it to take effect you will need to log out and back in - or at least, start a new login shell e.g. using

    su - username

or

    su -l username

Note that sudo should not be used to edit these personal files, as it will likely leave them owned by root, which can cause further problems down the road. If you have already used sudo vim you may need to use sudo chown to restore their correct ownership e.g.

sudo chown username:username ~/.profile

Then you can add the desired path component using your preferred editor e.g.

vim ~/.profile

You could even consider copying the existing paradigm for ~/bin in that file i.e. add it as

# set PATH so it includes user's private R bin if it exists
if [ -d "$HOME/R/bin" ] ; then
    PATH="$HOME/R/bin:$PATH"
fi