Fix: Set lf-activate-env to use Python 3.8

Set lf-activate-env to use Python 3.8 while running lftools deploy
logs. This fixes the below warnings which when jobs try to use
default version of python 3.6 which is EOL.

CryptographyDeprecationWarning: Python 3.6 is no longer supported
by the Python core team. Therefore, support for it is deprecated in
cryptography and will be removed in a future release.

PythonDeprecationWarning: Boto3 will no longer support Python 3.6
starting May 30, 2022. To continue receiving service updates, bug
fixes, and security updates please upgrade to Python 3.7 or later.

Issue-ID: RELENG-4348
Change-Id: I7177eda0afab58bd775801e955c8759c57554ff3
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>