build-sys: fix undefined ARCH error

../qga/meson.build:76:4: ERROR: Key ARCH is not in the dictionary.

Fixes commit 823eb013 ("configure, meson: move ARCH to meson.build")

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220114084312.3725242-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qga/meson.build

@@ -75,7 +75,7 @@ if targetos == 'windows'
     endif
     qga_msi = custom_target('QGA MSI',
                             input: files('installer/qemu-ga.wxs'),
-                            output: 'qemu-ga-@0@.msi'.format(config_host['ARCH']),
+                            output: 'qemu-ga-@0@.msi'.format(host_arch),
                             depends: deps,
                             command: [
                               find_program('env'),