patch 9.0.0582: channel cwd test fails on Cirrus CI

Problem:    Channel cwd test fails on Cirrus CI.
Solution:   Also remove /private from the expected directory.
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 5e1127e..a13b49f 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1989,8 +1989,13 @@
   let job = job_start(cmd, {'callback': {ch,msg -> execute(":let g:envstr .= msg")}, 'cwd': expect})
   try
     call WaitForAssert({-> assert_notequal("", g:envstr)})
+    " There may be a trailing slash or not, ignore it
     let expect = substitute(expect, '[/\\]$', '', '')
     let g:envstr = substitute(g:envstr, '[/\\]$', '', '')
+    " on CI there can be /private prefix or not, ignore it
+    if $CI != '' && stridx(expect, '/private/') == 0
+      let expect = expect[8:]
+    endif
     if $CI != '' && stridx(g:envstr, '/private/') == 0
       let g:envstr = g:envstr[8:]
     endif