fix(env): Do not override misc env set before sourcing oh-my-zsh (#5231)

Sourcing oh-my-zsh happens in zshrc, which will override settings of
profile and zshenv.  Treat misc values, `PAGER` and `LESS`, as default
settings without overriding existing values.

Fixes: #1, robbyrussell/oh-my-zsh#3016