[build.ps1] Fix newer PS Core compatibility

In Powershell Core, `$_` from `Get-ChildItem` output will be interpreted as full path. Use `$($_.Name)` to ensure only base file name is used.

Author: stevapple

utils/build.ps1

@@ -713,7 +713,7 @@ function Fetch-Dependencies {
     Get-ChildItem "$BinaryCache\toolchains\WixAttachedContainer" -Filter "*.msi" | ForEach-Object {
       $LogFile = [System.IO.Path]::ChangeExtension($_.Name, "log")
       $TARGETDIR = if ($_.Name -eq "rtl.msi") { "$BinaryCache\toolchains\$ToolchainName\LocalApp\Programs\Swift\Runtimes\$(Get-PinnedToolchainVersion)\usr\bin" } else { "$BinaryCache\toolchains\$ToolchainName" }
-    Invoke-Program -OutNull msiexec.exe /lvx! $BinaryCache\toolchains\$LogFile /qn /a $BinaryCache\toolchains\WixAttachedContainer\$_ ALLUSERS=0 TARGETDIR=$TARGETDIR
+    Invoke-Program -OutNull msiexec.exe /lvx! $BinaryCache\toolchains\$LogFile /qn /a $BinaryCache\toolchains\WixAttachedContainer\$($_.Name) ALLUSERS=0 TARGETDIR=$TARGETDIR
     }
   }