Don't use powershell to install python dependencies.

Signed-off-by: David Calavera <[email protected]>

Author: calavera

.appveyor.yml

@@ -80,8 +80,8 @@ for:
 
       # install Zig to cross compile Rust functions
       - ps: "$env:CARGO_ZIGBUILD_PYTHON_PATH = \"$env:PYTHON\\python.exe\""
-      - ps: "$env:CARGO_ZIGBUILD_PYTHON_PATH -m pip install ziglang"
-      - ps: "$env:CARGO_ZIGBUILD_PYTHON_PATH -m ziglang version"
+      - "%PYTHON%\\python.exe -m pip install ziglang"
+      - "%PYTHON%\\python.exe -m ziglang version"
 
       # install Rust
       - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe