autotests: Fix testenv script on Unix.

Summary:
Commit f89c0b56 ("Akonaditest: Fix testenv script on Windows") ended up
breking the shell script created on non-Windows environments.

Instead of

    old_foo="bar"

we were doing

    $old_foo="bar"

which then failed to be sourced.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D10491