patch 8.1.0050: $VIM_TERMINAL is also set when not in a terminal window

Problem:    $VIM_TERMINAL is also set when not in a terminal window.
Solution:   Pass a flag to indicate whether the job runs in a terminal.
diff --git a/src/terminal.c b/src/terminal.c
index 275a5a7..9b7bc03 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -5769,7 +5769,7 @@
 #endif
 
     /* This may change a string in "argvar". */
-    term->tl_job = job_start(argvar, argv, opt);
+    term->tl_job = job_start(argvar, argv, opt, TRUE);
     if (term->tl_job != NULL)
 	++term->tl_job->jv_refcount;