bug#61748: 27.2; Eglot should use shell-file-name when launching the language server for a remote file

[jeberger@free.fr]

From https://github.com/joaotavora/eglot/issues/1175

# Steps to reproduce:

- On the remote machine:
    - Install a language server in a non-standard path (e.g. `rust-analyzer`, which installs in a subdirectory under `$HOME` and not `/usr/bin`)
    - Make sure that the server is in the path for a shell other than `sh` (e.g. set the path in `.zprofile` for `zsh`).
- On the local machine:
    - Open a file remotely through TRAMP
    - Set `shell-file-name` to the shell you're using on the remote (e.g. `(setq shell-file-name "zsh")`)
    - <key>M-x</key> `eglot`.

# Expected result:

Eglot should start

# Actual result:

Eglot asks `Enter program to execute (or <host>:<port>):`, then when I type `rust-analyzer` it fails with message `[jsonrpc] Server exited with status 127`. Looking at `eglot-events-buffer`, it is clear that eglot tried to use `sh` to start the server.

# Partial fix

The server starts correctly if I change function `eglot--cmd` to use `(or shell-file-name "sh")` instead of plain `"sh"`, e.g.:

```lisp
(defun eglot--cmd (contact)
  "Helper for `eglot--connect'."
  (if (file-remote-p default-directory)
      ;; TODO: this seems like a bug, although it’s everywhere. For
      ;; some reason, for remote connections only, over a pipe, we
      ;; need to turn off line buffering on the tty.
      ;;
      ;; Not only does this seem like there should be a better way,
      ;; but it almost certainly doesn’t work on non-unix systems.
      (list (or shell-file-name "sh") "-c"
            (string-join (cons "stty raw > /dev/null;"
                               (mapcar #'shell-quote-argument contact))
                         " "))
    contact))
```

It still fails to autodetect the server and asks for the program to execute though.

----

In GNU Emacs 27.2 (build 1, x86_64-w64-mingw32)
 of 2021-03-26 built on CIRROCUMULUS
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19042
System Description: Microsoft Windows 10 Enterprise (v10.0.2009.19042.2604)

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