neovim-flake/docs/manual/configuring/languages/lsp.md

LSP Custom Packages/Command

In any of the opt.languages.<language>.lsp.package options you can provide your own LSP package, or provide the command to launch the language server, as a list of strings.

You can use this to skip automatic installation of a language server, and instead use the one found in your $PATH during runtime, for example:

vim.languages.java = {
  lsp = {
    enable = true;
    package = ["jdt-language-server" "-data" "~/.cache/jdtls/workspace"];
  };
}