sched/: Replace explict references to g_readytorun with indirect references via the this_task() macro

sched/environ/env_clearenv.c

@@ -74,7 +74,7 @@
 
 int clearenv(void)
 {
-  FAR struct tcb_s *tcb = (FAR struct tcb_s *)g_readytorun.head;
+  FAR struct tcb_s *tcb = this_task();
   DEBUGASSERT(tcb->group);
 
   env_release(tcb->group);

sched/environ/env_dup.c

@@ -81,7 +81,7 @@
 
 int env_dup(FAR struct task_group_s *group)
 {
-  FAR struct tcb_s *ptcb = (FAR struct tcb_s *)g_readytorun.head;
+  FAR struct tcb_s *ptcb = this_task();
   FAR char *envp = NULL;
   size_t envlen;
   int ret = OK;

sched/environ/env_getenv.c

@@ -94,7 +94,7 @@ FAR char *getenv(const char *name)
   /* Get a reference to the thread-private environ in the TCB. */
 
   sched_lock();
-  rtcb = (FAR struct tcb_s *)g_readytorun.head;
+  rtcb  = this_task();
   group = rtcb->group;
 
   /* Check if the variable exists */

sched/environ/env_getenvironptr.c

@@ -85,7 +85,7 @@ FAR char **get_environ_ptr(void)
 
   /* Return a reference to the thread-private environ in the TCB. */
 
-  FAR struct tcb_s *ptcb = (FAR struct tcb_s *)g_readytorun.head;
+  FAR struct tcb_s *ptcb = this_task();
   if (ptcb->envp)
     {
       return &ptcb->envp->ev_env;

sched/environ/env_setenv.c

@@ -124,7 +124,7 @@ int setenv(FAR const char *name, FAR const char *value, int overwrite)
   /* Get a reference to the thread-private environ in the TCB. */
 
   sched_lock();
-  rtcb  = (FAR struct tcb_s *)g_readytorun.head;
+  rtcb  = this_task();
   group = rtcb->group;
   DEBUGASSERT(group);
 

sched/environ/env_unsetenv.c

@@ -77,7 +77,7 @@
 
 int unsetenv(FAR const char *name)
 {
-  FAR struct tcb_s *rtcb = (FAR struct tcb_s *)g_readytorun.head;
+  FAR struct tcb_s *rtcb = this_task();
   FAR struct task_group_s *group = rtcb->group;
   FAR char *pvar;
   FAR char *newenvp;