fzf: fix patch for vim plugin; enable tests; avoid direct $src layout dependency

[bhipple]

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.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[bhipple]

@GrahamcOfBorg build fzf