Clean cargo cache after building

appveyor.yml

@@ -59,10 +59,9 @@ install:
 #  - ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
 
 build_script:
-  - set CARGO_HOME=C:\Users\appveyor\.cargo
   - mach clean-nightlies --keep 2 --force
-  - mach clean-cargo-cache --keep 2 --force
   - mach build -d -v
+  - mach clean-cargo-cache --keep 2 --force
   - mach test-unit
 
 test: off