keychain-environment: use temp-buffer to visit files

Using find-file-noselect has a lit of potential side effects including
running various hooks. Considering we only read two files when we
refresh the environment we can replace the read code with a very simple
with-temp-buffer form to do it for us.

Signed-off-by: Alex Bennée <alex@bennee.com>
Merge pull request #1 from stsquad/master

keychain-environment: use temp-buffer to visit files