Fix not resolving timezone aliases

Commit 2c763ac3 changed `readlink` to
use `--canonicalize-missing` but that resolves **all** symlink including
timezone aliases (e.g. `Arctic/Longyearbyen` points to `Europe/Berlin`).

Revert the code change from commit
2c763ac3 and use `realpath` instead.
This converts relative paths to absolute paths without resolving
symlinks inside the path.

Closes: #1030742
Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>