refactor: rename deployer to broadcaster

feat: use $ETH_FROM as broadcaster

script/Base.s.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity >=0.8.19;
+pragma solidity >=0.8.19 <=0.9.0;
 
 import { Script } from "forge-std/Script.sol";
 
@@ -10,19 +10,31 @@ abstract contract BaseScript is Script {
     /// @dev Needed for the deterministic deployments.
     bytes32 internal constant ZERO_SALT = bytes32(0);
 
-    /// @dev The address of the contract deployer.
-    address internal deployer;
+    /// @dev The address of the transaction broadcaster.
+    address internal broadcaster;
 
-    /// @dev Used to derive the deployer's address.
+    /// @dev Used to derive the broadcaster's address if $ETH_FROM is not defined.
     string internal mnemonic;
 
+    /// @dev Initializes the transaction broadcaster like this:
+    ///
+    /// - If $ETH_FROM is defined, use it.
+    /// - Otherwise, derive the broadcaster address from $MNEMONIC.
+    /// - If $MNEMONIC is not defined, default to a test mnemonic.
+    ///
+    /// The use case for $ETH_FROM is to specify the broadcaster key and its address via the command line.
     constructor() {
-        mnemonic = vm.envOr("MNEMONIC", TEST_MNEMONIC);
-        (deployer,) = deriveRememberKey({ mnemonic: mnemonic, index: 0 });
+        address from = vm.envOr({ name: "ETH_FROM", defaultValue: address(0) });
+        if (from != address(0)) {
+            broadcaster = from;
+        } else {
+            mnemonic = vm.envOr({ name: "MNEMONIC", defaultValue: TEST_MNEMONIC });
+            (broadcaster,) = deriveRememberKey({ mnemonic: mnemonic, index: 0 });
+        }
     }
 
-    modifier broadcaster() {
-        vm.startBroadcast(deployer);
+    modifier broadcast() {
+        vm.startBroadcast(broadcaster);
         _;
         vm.stopBroadcast();
     }

script/Deploy.s.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: UNLICENSED
-pragma solidity >=0.8.19;
+pragma solidity >=0.8.19 <=0.9.0;
 
 import { Foo } from "../src/Foo.sol";
 
@@ -7,7 +7,7 @@ import { BaseScript } from "./Base.s.sol";
 
 /// @dev See the Solidity Scripting tutorial: https://book.getfoundry.sh/tutorials/solidity-scripting
 contract Deploy is BaseScript {
-    function run() public broadcaster returns (Foo foo) {
+    function run() public broadcast returns (Foo foo) {
         foo = new Foo();
     }
 }