Fix onCooldown condition with "Show GCD" enabled

If we overwrite a spell's cooldown with the gcd cooldown, we should only
set the gcdCooldown flag if the spell is not on cooldown. As spell that
have the gcdCooldown flag set are considered not on cd

Fixes: #4621

WeakAuras/GenericTrigger.lua

@@ -2517,10 +2517,12 @@ do
 
     if (showgcd) then
       if ((gcdStart or 0) + (gcdDuration or 0) > startTime + duration) then
+        if startTime == 0 then
+          gcdCooldown = true
+        end
         startTime = gcdStart;
         duration = gcdDuration;
         modRate = gcdModrate
-        gcdCooldown = true;
       end
     end
 
@@ -2549,9 +2551,11 @@ do
     end
     if (showgcd) then
       if ((gcdStart or 0) + (gcdDuration or 0) > startTime + duration) then
+        if startTime == 0 then
+          gcdCooldown = true
+        end
         startTime = gcdStart;
         duration = gcdDuration;
-        gcdCooldown = true;
       end
     end
     return startTime, duration, enabled, gcdCooldown;
@@ -2583,9 +2587,11 @@ do
 
     if (showgcd) then
       if ((gcdStart or 0) + (gcdDuration or 0) > startTime + duration) then
+        if startTime == 0 then
+          gcdCooldown = true
+        end
         startTime = gcdStart;
         duration = gcdDuration;
-        gcdCooldown = true;
       end
     end
     return startTime, duration, enabled, gcdCooldown;