CI: Respect user-specified build directory

The Windows build script would respect the user-specified build
directory for the configure step, but not for the build step. Fix that
by applying the same logic in Configure-OBS to Build-OBS.

CI/windows/02_build_obs.ps1

@@ -41,7 +41,9 @@ function Build-OBS {
     Ensure-Directory ${CheckoutDir}
     Write-Step "Build OBS targets..."
 
-    Invoke-Expression "cmake --build $(if($BuildArch -eq "x64") { "build64" } else { "build32" }) --config ${BuildConfiguration}"
+    $BuildDirectoryActual = "${BuildDirectory}$(if (${BuildArch} -eq "x64") { "64" } else { "32" })"
+
+    Invoke-Expression "cmake --build ${BuildDirectoryActual} --config ${BuildConfiguration}"
 }
 
 function Configure-OBS {