GNUmakefile: Simplify the $SHELL

GNUmakefile: Simplify the $SHELL

nixpkgs does not provide /usr/bin/env, so they had to work around it by
specifying just env.  Since make(1) accepts program names instead of
paths, we can as well use bash directly, which is simpler.

Link: <https://github.com/NixOS/nixpkgs/pull/300797>
Reported-by: Sergei Trofimovich <slyich@gmail.com>
Cc: Mario Rodas <https://github.com/marsam>
Cc: Eli Schwartz <eschwartz93@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>