Fix USB creation when install.wim is read-only (#4910)

* fix: allow USB WIM splitting

* test: guard USB WIM splitting fix

functions/private/Invoke-WinUtilISOUSB.ps1

@@ -241,6 +241,7 @@ function Invoke-WinUtilISOWriteUSB {
                 $wimSizeMB = [math]::Round((Get-Item $installWim).Length / 1MB)
                 if ($wimSizeMB -gt 3800) {
                     Log "install.wim is $wimSizeMB MB - splitting for FAT32 compatibility... This will take several minutes."
+                    Set-ItemProperty -LiteralPath $installWim -Name IsReadOnly -Value $false
                     $splitDest = Join-Path $usbDrive "sources\install.swm"
                     New-Item -ItemType Directory -Path (Split-Path $splitDest) -Force
                     Split-WindowsImage -ImagePath $installWim -SplitImagePath $splitDest -FileSize 3800 -CheckIntegrity

pester/win11creator.Tests.ps1

@@ -191,6 +191,18 @@ Describe "Win11 Creator setup media" {
         $confirmationIndex | Should -BeGreaterThan $guardIndex
     }
 
+    It "clears install.wim read-only attribute before FAT32 splitting" {
+        $splitGuardIndex = $script:writeUsbFunction.IndexOf('$wimSizeMB -gt 3800')
+        $readOnlyResetIndex = $script:writeUsbFunction.IndexOf(
+            'Set-ItemProperty -LiteralPath $installWim -Name IsReadOnly -Value $false'
+        )
+        $splitCommandIndex = $script:writeUsbFunction.IndexOf('Split-WindowsImage')
+
+        $splitGuardIndex | Should -BeGreaterThan -1
+        $readOnlyResetIndex | Should -BeGreaterThan $splitGuardIndex
+        $splitCommandIndex | Should -BeGreaterThan $readOnlyResetIndex
+    }
+
     It "maps Windows edition names to setup edition IDs" {
         . ([scriptblock]::Create($script:editionIdFunction))