Auto merge of #21833 - servo:jdm-patch-37, r=SimonSapin

Fix appveyor build

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21832
- [x] There are tests for these changes

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appveyor.yml

@@ -55,8 +55,8 @@ install:
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
   - set PKG_CONFIG_PATH=%PKG_CONFIG_PATH%;C:\gstreamer\1.0\x86_64\lib\pkgconfig
   - rustup -V
-  - mach rustc --version
-  - mach cargo --version
+  - rustc --version
+  - cargo --version
 
 # Uncomment these lines to expose RDP access information to the build machine in the build log.
 #init: