kbuild: Allow to select bash in a modified environment

This fixes the build error when the system has a default bash version
which is too old to support associative array variables.

The build error log as fellowing:
linux/scripts/check-local-export: line 11: declare: -A: invalid option
declare: usage: declare [-afFirtx] [-p] [name[=value] ...]

Signed-off-by: Schspa Shi <schspa@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>