fzf: fix patch for vim plugin; enable tests; avoid direct $src… (#79575)

It was previously referencing `$bin`, but this package no longer produces a
`bin` output, just `out` and `share`. Updated to make the comparison check a bit
more robust.

Also updated to avoid direct dependency on the `$src` directory out of the nix
store, instead using the processed src setup in the unpackPhase. This provides a
cleaner abstraction between the build/install phase and the input src phase, and
avoids an unnecessary dependency on whether the source disted tarball comes from
`fetchFromGitHub` (in which case it's an unpacked directory) or something like
`fetchurl`. In either case, stdenv is responsible for processing the input `src`
and setting up a clean build dir for us, so we should use that.

This produces an equivalent directory tree, except that the vim plugin is no
longer broken.