refactor: change value of nextCastTime (#6)

Make it consistent with other emergency spells.

src/EmergencyDropSpell.sol

@@ -64,9 +64,9 @@ contract EmergencyDropSpell is EmergencySpellLike {
     uint256 public immutable eta;
     /// @notice The original spell encoded call.
     bytes public sig;
-    /// @notice Emergency spells can be cast immediately after deployed.
-    /// @dev Declared to keep compatibility with `DssExec`.
-    uint256 public immutable nextCastTime = block.timestamp;
+    // @dev An emergency spell does not need to be cast, as all actions happen during the schedule phase.
+    //      Notice that cast is usually not supposed to revert, so it is implemented as a no-op.
+    uint256 public immutable nextCastTime = type(uint256).max;
 
     /// @notice Drop have been called.
     event Drop();