libnixf(sema-primop-unknown): false-positive for experimental features

[MattSturgeon]

Describe the bug

When linting builtins.getFlake, getFlake is flagged as an unknown builtin. I assume this is because it is gated behind experimental-features=flakes.

[sema-primop-unknown] Error: unknown builtin `getFlake`
    ╭─[nvim/config/_nixd-expr.nix:84:21]
    │
 84 │   global = builtins.getFlake self;
    ·                     ────┬───
    ·                         ╰───── unknown builtin `getFlake`
────╯

$ nix repl
Nix 2.31.2+1
Type :? for help.
nix-repl> builtins.getFlake
«primop getFlake»

Additional context
"impure" features don't seem to false-positive, e.g. builtins.currentSystem is not flagged. I've only reproduced this with getFlake, so far.

[inclyc]

ACK