$(shell ...) expands to the output of the command. It expands to the
empty string when the command does not print anything to stdout.
Hence, $(shell mkdir ...) is sufficient and does not need any
variable assignment in front of it.

Commit c2bd08ba ("treewide: remove meaningless assignments in
Makefiles", 2024-02-23) did this to all of tools/ but ignored in-flight
changes to tools/testing/selftests/kvm/Makefile, so reapply the change.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>