package: Make Windows installer respect customized path (#41653)

Create a Burn variable and pass it to MSI, in order to really install
the bundle on customized path chosen in UI.

Doc:
https://docs.firegiant.com/wix/whatsnew/#default-installation-folder

Testing: Tested with
[try](https://github.com/servo/servo/pull/41653#issuecomment-3707098782).
- Install works
- Uninstall works
- Default path works

Fixes: #40995

---------

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

support/windows/Servo.wxs

@@ -3,6 +3,12 @@
   <Bundle Name="Servo"
           Version="1.0"
           UpgradeCode="91b09c7e-6c0d-4166-b806-1dc724acf728">
+
+    <Variable Name="InstallFolder" 
+          Type="string" 
+          Value="[ProgramFiles64Folder]Servo\Servo Tech Demo" 
+          bal:Overridable="yes" />
+
     <BootstrapperApplicationRef Id="WixStandardBootstrapperApplication.HyperlinkLicense">
       <bal:WixStandardBootstrapperApplication
         LicenseUrl="https://www.mozilla.org/en-US/MPL/2.0/"
@@ -13,7 +19,9 @@
           SourceFile="Installer.msi"
           Compressed="yes"
           DisplayName="Servo"
-          ForcePerMachine="yes"/>
+          ForcePerMachine="yes">
+          <MsiProperty Name="INSTALLDIR" Value="[InstallFolder]" />
+      </MsiPackage>
     </Chain>
   </Bundle>
 </Wix>