utils: use cmark gfm for the build when running the compiler

When using the just built compiler, we need to use the build CMark
runtime as it is executing on the build. This used to work when host and
build were guaranteed to be identical (i.e. no cross-compilation). This
is now needed to support cross-compiling ARM64.

(cherry picked from commit 421c0cf)

Author: compnerd

utils/build.ps1

@@ -1350,7 +1350,7 @@ function Build-ICU([Platform]$Platform, $Arch) {
 
 function Build-Runtime([Platform]$Platform, $Arch) {
   Isolate-EnvVars {
-    $env:Path = "$($HostArch.BinaryCache)\cmark-gfm-0.29.0.gfm.13\src;$(Get-PinnedToolchainRuntime);${env:Path}"
+    $env:Path = "$($BuildArch.BinaryCache)\cmark-gfm-0.29.0.gfm.13\src;$(Get-PinnedToolchainRuntime);${env:Path}"
 
     $CompilersBinaryCache = if ($IsCrossCompiling) {
       Get-BuildProjectBinaryCache Compilers