Get rid of `lazy_static!` blocks

[Radvendii]

Nix init functions (nix_libstore_init() and nix_libexpr_init()) can be called multiple times and are idempotent. This means we don't need to guarantee ourselves that they're only called once through the use of lazy_static!

[prednaz]

a function being idempotent does not mean that calling it a second time is cheap.

[prednaz]

however, in the case of nix_libstore_init idempotence does mean that calling it a second time is cheap because idempotence is achieved by the line if (initLibStoreDone) return;. but this is not thread safe. i guess, that is why @roberth made use of the thread safe flavor of mutable static variables that lazy_static provides and added a /* TODO make Nix itself thread safe */ comment.