Fix o.shell_succeeds() always returning false inside Hyprland

Hyprland reaps its own children, so os.execute() gets ECHILD from waitpid and
never sees an exit status. Every call reported failure, which meant the NVIDIA
env detection in hypr/nvidia.lua never set NVD_BACKEND, LIBVA_DRIVER_NAME or
__GLX_VENDOR_LIBRARY_NAME. Read a marker off stdout instead.

Closes #6914

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

default/hypr/helpers.lua

@@ -18,9 +18,20 @@ local function file_exists(path)
   return false
 end
 
+-- Hyprland reaps its own children, so os.execute() can't retrieve an exit status
+-- from inside the compositor. Read a marker off stdout instead.
 function o.shell_succeeds(command)
-  local ok, _, code = os.execute(command .. " >/dev/null 2>&1")
-  return ok == true or ok == 0 or code == 0
+  -- Subshell, so the redirection covers every command rather than binding to
+  -- the last one and letting an earlier one write its own OK into the pipe.
+  local pipe = io.popen("( " .. command .. " ) >/dev/null 2>&1 && echo OK")
+  if not pipe then
+    return false
+  end
+
+  local output = pipe:read("*a") or ""
+  pipe:close()
+
+  return output:find("OK", 1, true) ~= nil
 end
 
 function o.cmd_present(command)