CI: fix nightly build

[japaric]

linked_list_allocator (LLA) added a default (opt-out) Cargo feature, "const_mut_refs", in 0.8.7. This feature controls whether Heap::new is a const function or not.
alloc-cortex-m uses linked_list_allocator with default-features = no
as a result alloc-cortex-m doesn't compile on nightly after 0.8.7 was released

to work around the issue we manually enable the LLA's "const_mut_refs"