runtime(vim9): remove unnecessary temp variable

closes: #20374

Signed-off-by: Mao-Yining <mao.yining@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/autoload/dist/vim9.vim

@@ -3,7 +3,7 @@ vim9script
 # Vim runtime support library
 #
 # Maintainer:   The Vim Project <https://github.com/vim/vim>
-# Last Change:  2026 May 06
+# Last Change:  2026 May 30
 
 export def IsSafeExecutable(filetype: string, executable: string): bool
   if empty(exepath(executable))
@@ -131,13 +131,12 @@ enddef
 export def Open(file: string)
   # disable shellslash for shellescape, required on Windows #17995
   if exists('+shellslash') && &shellslash
-    &shellslash = false
     defer setbufvar('%', '&shellslash', true)
+    &shellslash = false
   endif
   if &shell == 'pwsh' || &shell == 'powershell'
-    const shell = &shell
+    defer setbufvar('%', '&shell', &shell)
     setlocal shell&
-    defer setbufvar('%', '&shell', shell)
   endif
   if has('unix') && !has('win32unix') && !exists('$WSL_DISTRO_NAME')
     Launch($"{Viewer()} {shellescape(file)}")