[Concurrency] Fix a typo and use var not let.

Fix a typo that caused us to not include the correct code for platforms
that use "singlethreaded" concurrency.

Also use `var` not `let` for the computed property in `PlatformExecutorFactory`.

rdar://141348916

Author: al45tair

stdlib/public/Concurrency/CMakeLists.txt

@@ -177,7 +177,7 @@ if("${SWIFT_CONCURRENCY_GLOBAL_EXECUTOR}" STREQUAL "dispatch")
     ExecutorImpl.swift
   )
 elseif("${SWIFT_CONCURRENCY_GLOBAL_EXECUTOR}" STREQUAL "singlethreaded")
-  set(SWIFT_RUNTIME_CONCURRENCYU_NONEMBEDDED_SWIFT_SOURCES
+  set(SWIFT_RUNTIME_CONCURRENCY_NONEMBEDDED_SWIFT_SOURCES
     ExecutorImpl.swift
     PlatformExecutorCooperative.swift
     )

stdlib/public/Concurrency/PlatformExecutorCooperative.swift

@@ -16,6 +16,6 @@ import Swift
 @available(SwiftStdlib 6.2, *)
 public struct PlatformExecutorFactory: ExecutorFactory {
   static let executor = CooperativeExecutor()
-  public static let mainExecutor: any MainExecutor { executor }
-  public static let defaultExecutor: any TaskExecutor { executor }
+  public static var mainExecutor: any MainExecutor { executor }
+  public static var defaultExecutor: any TaskExecutor { executor }
 }