autotests: Fix testenv script on Unix.

Summary:
Commit f89c0b569 ("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