[skim v2] avoid panic when cache is disabled

[cdmistman]

when i set .use_cache(false) i got a panic:

thread 'my-thread' panicked at /HOME/.cargo/registry/src/index.crates.io-6f17d22bba15001f/fuzzy-matcher-0.3.7/src/skim.rs:932:48:
already borrowed: BorrowMutError
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

i think that's because m is already an owned borrow (line 837), and same with choice_chars and pattern_chars, all of which are still owned when we get to this if

disclaimer: i didn't actually test this change (i did it in github's inline editor lmao). i can test later if desired