patch 8.1.1694: the RUN_VIM variable is longer than needed

Problem:    The RUN_VIM variable is longer than needed.
Solution:   Shorten RUN_VIM. (Daniel Hahler, closes #4643)
diff --git a/src/testdir/shared.vim b/src/testdir/shared.vim
index e1bb818..9e4b1bb 100644
--- a/src/testdir/shared.vim
+++ b/src/testdir/shared.vim
@@ -261,7 +261,7 @@
     let cmd = cmd . ' -u ' . name
   endif
   let cmd .= ' --not-a-term'
-  let cmd = substitute(cmd, 'VIMRUNTIME=.*VIMRUNTIME;', '', '')
+  let cmd = substitute(cmd, 'VIMRUNTIME=\S\+', '', '')
 
   " If using valgrind, make sure every run uses a different log file.
   if cmd =~ 'valgrind.*--log-file='