Add Slither reentrancy check in CI (#3047)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/governance/TimelockController.sol

@@ -261,6 +261,9 @@ contract TimelockController is AccessControl {
      *
      * - the caller must have the 'executor' role.
      */
+    // This function can reenter, but it doesn't pose a risk because _afterCall checks that the proposal is pending,
+    // thus any modifications to the operation during reentrancy should be caught.
+    // slither-disable-next-line reentrancy-eth
     function execute(
         address target,
         uint256 value,

contracts/governance/extensions/GovernorTimelockControl.sol

@@ -122,6 +122,9 @@ abstract contract GovernorTimelockControl is IGovernorTimelock, Governor {
      * @dev Overriden version of the {Governor-_cancel} function to cancel the timelocked proposal if it as already
      * been queued.
      */
+    // This function can reenter through the external call to the timelock, but we assume the timelock is trusted and
+    // well behaved (according to TimelockController) and this will not happen.
+    // slither-disable-next-line reentrancy-no-eth
     function _cancel(
         address[] memory targets,
         uint256[] memory values,

contracts/token/ERC20/extensions/ERC20FlashMint.sol

@@ -56,6 +56,9 @@ abstract contract ERC20FlashMint is ERC20, IERC3156FlashLender {
      * @param data An arbitrary datafield that is passed to the receiver.
      * @return `true` is the flash loan was successful.
      */
+    // This function can reenter, but it doesn't pose a risk because it always preserves the property that the amount
+    // minted at the beginning is always recovered and burned at the end, or else the entire function will revert.
+    // slither-disable-next-line reentrancy-no-eth
     function flashLoan(
         IERC3156FlashBorrower receiver,
         address token,