utils: update toolchain snapshot to permit use of the experimental SDK

Use the experimental SDK to build the toolchain and consume the
redistributable. This is going to ensure that we build the entire
toolchain against a stable point and run against the runtime that we
build against.

Author: compnerd

utils/build.ps1

@@ -269,14 +269,14 @@ if ($UseHostToolchain -is [string]) {
 
 $DefaultPinned = @{
   AMD64 = @{
-    PinnedBuild = "https://download.swift.org/swift-6.1.2-release/windows10/swift-6.1.2-RELEASE/swift-6.1.2-RELEASE-windows10.exe";
-    PinnedSHA256 = "92A0323ED7DD333C3B05E6E0E428F3A91C77D159F6CCFC8626A996F2ACE09A0B";
-    PinnedVersion = "6.1.2";
+    PinnedBuild = "https://ci-external.swift.org/job/swift-main-windows-toolchain/1656/artifact/build/artifacts/installer.exe";
+    PinnedSHA256 = "13841E44438D4EF1C40544B5AD97B8D549C0B2DA0E66BAEC5039EE48404D0D9A";
+    PinnedVersion = "0.0.0";
   };
   ARM64 = @{
-    PinnedBuild = "https://download.swift.org/swift-6.1.2-release/windows10-arm64/swift-6.1.2-RELEASE/swift-6.1.2-RELEASE-windows10-arm64.exe";
-    PinnedSHA256 = "121FB407E578178F82DCCF39A4D03527873D8F7611A801A8FC26DA52503A0C5C";
-    PinnedVersion = "6.1.2";
+    PinnedBuild = "https://download.swift.org/development/windows10-arm64/swift-DEVELOPMENT-SNAPSHOT-2025-09-21-a/swift-DEVELOPMENT-SNAPSHOT-2025-09-21-a-windows10-arm64.exe"
+    PinnedSHA256 = "";
+    PinnedVersion = "0.0.0";
   };
 }
 
@@ -2205,6 +2205,7 @@ function Get-CompilersDefines([Hashtable] $Platform, [string] $Variant, [switch]
   return $TestDefines + $DebugDefines + @{
     CLANG_TABLEGEN = (Join-Path -Path $BuildTools -ChildPath "clang-tblgen.exe");
     CLANG_TIDY_CONFUSABLE_CHARS_GEN = (Join-Path -Path $BuildTools -ChildPath "clang-tidy-confusable-chars-gen.exe");
+    CMAKE_STATIC_LIBRARY_PREFIX_Swift = "lib";
     CMAKE_Swift_FLAGS = $SwiftFlags;
     LibXml2_DIR = "$BinaryCache\$($Platform.Triple)\usr\lib\cmake\libxml2-2.11.5";
     LLDB_LIBXML2_VERSION = "2.11.5";