Autobuild: Ensure JACK install completion

Previously, the JACK installers were started as ordinary programs. As
they are GUI applications, they detach from the console and the
powershell build script (and Github Actions) continues.
Therefore, once the actual build starts, the JACK installation may not
have finished at all.
This commit uses Powershell's Start-Process -Wait feature to wait
for completion before continuing.

Fixes: #3035

.github/autobuild/windows.ps1

@@ -183,7 +183,7 @@ Function Ensure-JACK
 
     $JACKInstallPath = "${DownloadCacheDir}\JACK64.exe"
 
-    & $JACKInstallPath $JACKInstallParms
+    Start-Process -Wait $JACKInstallPath -ArgumentList "$JACKInstallParms"
 
     if ( !$? )
     {
@@ -198,7 +198,7 @@ Function Ensure-JACK
 
     $JACKInstallPath = "${DownloadCacheDir}\JACK32.exe"
 
-    & $JACKInstallPath $JACKInstallParms
+    Start-Process -Wait $JACKInstallPath -ArgumentList "$JACKInstallParms"
 
     if ( !$? )
     {