nvim: use tool update instead of tool install

dotnet is funny. `tool install` fails if the tool is already installed,
and the upstream issue (dotnet/sdk#9500) recommends using `update`
instead of `install`.

nvim/scripts/bootstrap.py

@@ -278,7 +278,7 @@ async def install_fsautocomplete() -> None:
         print("skipping fsautocomplete")
         return
 
-    await run_cmd("dotnet", ["tool", "install", "--global", "fsautocomplete"])
+    await run_cmd("dotnet", ["tool", "update", "--global", "fsautocomplete"])
 
 
 async def setup_langservers(cache_dir: Path) -> None: