docs: use build/bin/nvim instead of nvim in gen_vimdoc (#22398)

Problem:

`nvim` could point to stable release missing the `nvim -l`
functionality.

Solution:

Require to build nvim first and use `build/bin/nvim`

scripts/gen_vimdoc.py

@@ -42,6 +42,7 @@ import subprocess
 import collections
 import msgpack
 import logging
+from pathlib import Path
 
 from xml.dom import minidom
 
@@ -55,19 +56,19 @@ if sys.version_info < MIN_PYTHON_VERSION:
 doxygen_version = tuple((int(i) for i in subprocess.check_output(["doxygen", "-v"],
                         universal_newlines=True).split()[0].split('.')))
 
-# Until 0.9 is released, need this hacky way to check that "nvim -l foo.lua" works.
-nvim_version = list(line for line in subprocess.check_output(['nvim', '-h'], universal_newlines=True).split('\n')
-                     if '-l ' in line)
-
 if doxygen_version < MIN_DOXYGEN_VERSION:
     print("\nRequires doxygen {}.{}.{}+".format(*MIN_DOXYGEN_VERSION))
     print("Your doxygen version is {}.{}.{}\n".format(*doxygen_version))
     sys.exit(1)
 
-if len(nvim_version) == 0:
-    print("\nRequires 'nvim -l' feature, see https://github.com/neovim/neovim/pull/18706")
+
+# Need a `nvim` that supports `-l`, so use the local build
+nvim = Path(__file__).parent / "../build/bin/nvim"
+if not nvim.exists():
+    print("\nYou need to build Neovim first to build the documentation.")
     sys.exit(1)
 
+
 # DEBUG = ('DEBUG' in os.environ)
 INCLUDE_C_DECL = ('INCLUDE_C_DECL' in os.environ)
 INCLUDE_DEPRECATED = ('INCLUDE_DEPRECATED' in os.environ)
@@ -1163,7 +1164,7 @@ def main(doxygen_config, args):
 def filter_source(filename):
     name, extension = os.path.splitext(filename)
     if extension == '.lua':
-        p = subprocess.run(['nvim', '-l', lua2dox, filename], stdout=subprocess.PIPE)
+        p = subprocess.run([str(nvim), '-l', lua2dox, filename], stdout=subprocess.PIPE)
         op = ('?' if 0 != p.returncode else p.stdout.decode('utf-8'))
         print(op)
     else: