pallet-contracts: Fix compilation on latest nightly (paritytech#2986)

Apparently they changed detection for visibility identifiers on traits,
which broke more than it should. There is an issue open:
rust-lang/rust#119924 The easy solution for us
is to move the declaration of the global variable outside of the trait.

Closes: paritytech#2960
(cherry picked from commit 0e124a0)

substrate/frame/contracts/src/lib.rs

@@ -1153,6 +1153,9 @@ struct InternalOutput<T: Config, O> {
 	result: Result<O, ExecError>,
 }
 
+// Set up a global reference to the boolean flag used for the re-entrancy guard.
+environmental!(executing_contract: bool);
+
 /// Helper trait to wrap contract execution entry points into a single function
 /// [`Invokable::run_guarded`].
 trait Invokable<T: Config>: Sized {
@@ -1168,9 +1171,6 @@ trait Invokable<T: Config>: Sized {
 	/// We enforce a re-entrancy guard here by initializing and checking a boolean flag through a
 	/// global reference.
 	fn run_guarded(self, common: CommonInput<T>) -> InternalOutput<T, Self::Output> {
-		// Set up a global reference to the boolean flag used for the re-entrancy guard.
-		environmental!(executing_contract: bool);
-
 		let gas_limit = common.gas_limit;
 
 		// Check whether the origin is allowed here. The logic of the access rules