tests/utils/env.sh.in: make it portable for Bash and Zsh

tests/utils/env.sh.in: make it portable for Bash and Zsh

The syntax

    ${!varname:-}

isn't valid in Zsh.

To make it possible to source the generated `env.sh` interactively, make
this (more) portable.

Now _set_var_def() also sets if the variable is set but empty, but I
don't think it's an issue here.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I07d49199c403ac24d569d383fd8bde2cf5b668e6
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11769