Fix build failure on Swift Playgrounds

The PR #311 introduced `Context.environment`, so that
swift-async-algorithm can no longer be used in the Swift Playgrounds.

However, `Context.environment` is equivalent to
`ProcessInfo.processInfo.environment`, so this is an unnecessary
sacrifice.

This change is intended to fix that.

Author: kkebo

Package.swift

@@ -1,5 +1,6 @@
 // swift-tools-version: 6.2
 
+import Foundation
 import PackageDescription
 import CompilerPluginSupport
 
@@ -93,7 +94,7 @@ let package = Package(
   ]
 )
 
-if Context.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
+if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
   package.dependencies += [
     .package(url: "https://github.com/apple/swift-collections.git", from: "1.1.0")
   ]

Package@swift-5.8.swift

@@ -1,5 +1,6 @@
 // swift-tools-version: 5.8
 
+import Foundation
 import PackageDescription
 import CompilerPluginSupport
 
@@ -87,7 +88,7 @@ let package = Package(
   ]
 )
 
-if Context.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
+if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
   package.dependencies += [
     .package(url: "https://github.com/apple/swift-collections.git", from: "1.1.0")
   ]