tests: fix unbound variables in env.sh

tests: fix unbound variables in env.sh

When running a bash test script manually, the '_set_var_def' function
expands the variables using their simple form resulting in an unbound
variable error if they are undefined. Use the Default values form
instead in the eval to fix these errors:

    tests/utils/env.sh: line 12: BT_TESTS_AWK_BIN: unbound variable
    tests/utils/env.sh: line 12: BT_TESTS_GREP_BIN: unbound variable
    tests/utils/env.sh: line 12: BT_TESTS_PYTHON_BIN: unbound variable
    tests/utils/env.sh: line 12: BT_TESTS_PYTHON_CONFIG_BIN: unbound variable
    tests/utils/env.sh: line 12: BT_TESTS_SED_BIN: unbound variable
    tests/utils/env.sh: line 12: BT_TESTS_CC_BIN: unbound variable
    tests/utils/env.sh: line 12: BT_TESTS_ENABLE_ASAN: unbound variable
    tests/utils/env.sh: line 12: BT_TESTS_ENABLE_PYTHON_PLUGINS: unbound variable

Change-Id: Ic4b2a3b97bd6a4d776ab6898959742ef77988332
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11999
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>