cli/babeltrace.c: update known log level environment variable names

Eventually we could find another way to synchronize the environment
variable names we use throughout the project with the CLI. We want to
decouple the plugins from the CLI (plugins do not know CLI, but CLI
knows plugins) so I don't want the plugins to support an environment
variable defined by the CLI for example.

For the moment, I'm using this:

    rg --pcre2 -N --iglob='*.c' --iglob='*.h' --no-filename -o \
       'BABELTRACE_\w+(?<!CLI)_LOG_LEVEL\b' --color=never | sort | uniq

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I148c5c859570d0b1a654c829b31066155e8ecf5e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1258
Tested-by: jenkins