Bump swift-system to 1.2.1

So we can have one set of update-checkout changes

Requires swiftlang/swift-tools-support-core#358

Author: yim-lee

Package.swift

@@ -724,7 +724,7 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
         .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMinor(from: "1.2.2")),
         .package(url: "https://github.com/apple/swift-driver.git", branch: relatedDependenciesBranch),
         .package(url: "https://github.com/apple/swift-crypto.git", .upToNextMinor(from: "2.5.0")),
-        .package(url: "https://github.com/apple/swift-system.git", .upToNextMinor(from: "1.1.1")),
+        .package(url: "https://github.com/apple/swift-system.git", .upToNextMinor(from: "1.2.1")),
         .package(url: "https://github.com/apple/swift-collections.git", .upToNextMinor(from: "1.0.1")),
         .package(url: "https://github.com/apple/swift-certificates.git", .upToNextMinor(from: "0.6.0")),
     ]