Add SHA256 hash verification

Amended 3 times because of my stupidity...

appveyor.yml

@@ -11,6 +11,7 @@ environment:
   VULKAN_SDK: "C:\\VulkanSDK\\1.1.73.0"
   VULKAN_SDK_URL: https://sdk.lunarg.com/sdk/download/1.1.73.0/windows/VulkanSDK-1.1.73.0-Installer.exe
   VULKAN_SDK_MIRROR: https://obk.ee/rpcs3/VulkanSDK-1.1.73.0-Installer.exe
+  VULKAN_SDK_SHA: a5d193f97db4de97e6b4fdd81f00ff6a603f66bb17dc3cf8ac0fe9aec58497c7
 
 cache:
 - glslang.7z -> appveyor.yml
@@ -66,26 +67,28 @@ build:
   verbosity: normal
 
 before_build:
-- ps: | # fetch precompiled build dependencies
+- ps: | # initialize mirror arrays and fetch precompiled build dependencies
+    $VULKAN_SDK_URLS = @(
+        $env:VULKAN_SDK_MIRROR;
+        $env:VULKAN_SDK_URL;
+        "$($env:VULKAN_SDK_URL)?Human=true";
+    )
     if (!(test-path llvmlibs.7z)) { irm $env:LLVMLIBS -outfile llvmlibs.7z }
     if (!(test-path glslang.7z)) { irm $env:GLSLANG -outfile glslang.7z }
     7z x llvmlibs.7z -aos -o"." | out-null
     7z x glslang.7z -aos -o".\lib\$env:CONFIGURATION-$env:PLATFORM" | out-null
     if (!(test-path vulkan-sdk.exe)) 
-    { 
-        Try
-        {
-            irm $env:VULKAN_SDK_MIRROR -outfile vulkan-sdk.exe 
-        }
-        Catch
-        {
-            Try
+    {
+        foreach ($url in $VULKAN_SDK_URLS) {
+            irm $url -outfile vulkan-sdk.exe 
+            if ($env:VULKAN_SDK_SHA -eq $(Get-FileHash .\vulkan-sdk.exe).Hash) 
             {
-                irm $env:VULKAN_SDK_URL -outfile vulkan-sdk.exe 
+                echo "Successfully downloaded and verified vulkan-sdk.exe!"
+                break
             }
-            Catch
+            else 
             {
-                irm "$($env:VULKAN_SDK_URL)?Human=true" -outfile vulkan-sdk.exe 
+                echo "Invalid file hash, Vulkan SDK Setup was possibly tampered with, retrying with a different mirror..."
             }
         }
     }