Fix the mimalloc packaging error for arm64

Author: hjyamauchi

.github/workflows/swift-toolchain.yml

@@ -3244,7 +3244,7 @@ jobs:
               -p:PASSPHRASE=${{ secrets.PASSPHRASE }} `
               -p:DEVTOOLS_ROOT=${{ github.workspace }}/BuildRoot/Library/Developer/Toolchains/unknown-Asserts-development.xctoolchain `
               -p:TOOLCHAIN_ROOT=${{ github.workspace }}/BuildRoot/Library/Developer/Toolchains/unknown-Asserts-development.xctoolchain `
-              -p:ENABLE_MIMALLOC=true `
+              -p:ENABLE_MIMALLOC=${{ matrix.arch == 'amd64' && 'true' || 'false' }} `
               -p:ProductArchitecture=${{ matrix.arch }} `
               -p:ProductVersion=${{ inputs.swift_version }} `
               ${{ github.workspace }}/SourceCache/swift-installer-scripts/platforms/Windows/bld/bld.wixproj