backport: fix(clipboard): use tmux only in a tmux session (#36603)

backport #36407

This reverts 2495e7e. That past change meant that we would modify the
buffer contents of a tmux session if it exists, even if the current Nvim
process wasn't running inside of it. Depending on the tmux
configuration, this could even affect the clipboard of an actually
attached tmux client, since tmux itself uses OSC 52 to forward buffer
writes to attached clients.

While autodetection is usually a trade-off and can rarely make everybody
happy, this behavior goes counter the principle of least surprise. If
really desired, it can be brought back by explicit configuration.

Co-authored-by: Daniel Danner <dnnr@users.noreply.github.com>

runtime/autoload/provider/clipboard.vim

@@ -255,7 +255,7 @@ function! provider#clipboard#Executable() abort
     return s:set_clip()
   elseif executable('termux-clipboard-set')
     return s:set_termux()
-  elseif executable('tmux') && (!empty($TMUX) || 0 == jobwait([jobstart(['tmux', 'list-buffers'])], 2000)[0])
+  elseif !empty($TMUX) && executable('tmux')
     return s:set_tmux()
   elseif get(get(g:, 'termfeatures', {}), 'osc52') && &clipboard ==# ''
     " Don't use OSC 52 when 'clipboard' is set. It can be slow and cause a lot

runtime/doc/news.txt

@@ -323,6 +323,7 @@ OPTIONS
 • 'winborder' sets the default border for |floating-windows|.
 • 'winborder' "bold" style.
 • |g:clipboard| accepts a string name to force any builtin clipboard tool.
+• |g:clipboard| autodetection only selects tmux when running inside tmux
 
 PERFORMANCE