Makefile: Remove /usr/bin/env wrapper from the SHELL variable

Makefile: Remove /usr/bin/env wrapper from the SHELL variable

The wrapper should not be needed here (it's not the shebang line of
a shell script), and it is causing trouble on Haiku where "env"
resides in a different directory.

Reported-by: Richard Zak <richard.j.zak@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210705082542.936856-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>