pyoxidizer: pass arch to WiXInstaller()

We just upgraded Windows automation to PyOxidizer 0.17.0. This
version of PyOxidizer changed the behavior of WiX installers so
installer architecture defaulted to "x64" (before it defaulted
to the target of the pyoxidizer.exe binary). To allow controlling
the architecture of the installer, the `arch` argument was added
to `WiXInstaller`. This commit passes that argument in.

Differential Revision: https://phab.mercurial-scm.org/D11358

rust/hgcli/pyoxidizer.bzl

@@ -255,7 +255,7 b' def make_msi(manifest):'
         path = "defaultrc/editor.rc",
     )
 
-    wix = WiXInstaller("hg", "%s-%s.msi" % (MSI_NAME, VERSION))
+    wix = WiXInstaller("hg", "%s-%s.msi" % (MSI_NAME, VERSION), arch = platform)
 
     # Materialize files in the manifest to the install layout.
     wix.add_install_files(manifest)