need to unset MIRI_SYSROOT before calling 'cargo miri setup'

Author: RalfJung

.appveyor.yml

@@ -53,6 +53,7 @@ test_script:
     - cd test-cargo-miri
     - '"C:\msys64\mingw64\bin\python3.exe" run-test.py'
     - cd ..
+    - ps: $env:MIRI_SYSROOT = ""
     # Test foreign miri: 64bit Linux
     - cargo miri setup --target x86_64-unknown-linux-gnu
     - set MIRI_SYSROOT=%USERPROFILE%\AppData\Local\rust-lang\miri\cache
@@ -61,6 +62,7 @@ test_script:
     - cd test-cargo-miri
     - '"C:\msys64\mingw64\bin\python3.exe" run-test.py'
     - cd ..
+    - ps: $env:MIRI_SYSROOT = ""
     # Test foreign miri: 64bit macOS
     - cargo miri setup --target x86_64-apple-darwin
     - set MIRI_SYSROOT=%USERPROFILE%\AppData\Local\rust-lang\miri\cache
@@ -69,6 +71,7 @@ test_script:
     - cd test-cargo-miri
     - '"C:\msys64\mingw64\bin\python3.exe" run-test.py'
     - cd ..
+    - ps: $env:MIRI_SYSROOT = ""
 
 after_test:
     # Don't cache "master" toolchain, it's a waste