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/evalfunc.c b/src/evalfunc.c
index 3cd3c3d..2bbeb25 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -7254,7 +7254,7 @@
     rettv->v_type = VAR_JOB;
     if (check_restricted() || check_secure())
 	return;
-    rettv->vval.v_job = job_start(argvars, NULL, NULL);
+    rettv->vval.v_job = job_start(argvars, NULL, NULL, FALSE);
 }
 
 /*