chore: update appveyor.yaml

appveyor.yml

@@ -8,14 +8,16 @@ branches:
 environment:
   VCVARS: C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat
   VCVARSARG: x64
+  MINICONDA: C:\Miniconda3
 
 build: false
 
-before_test:
+install:
+  - "set PATH=%MINICONDA%;%MINICONDA%\\Scripts;%PATH%"
   - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init -yv --default-toolchain nightly --default-host x86_64-pc-windows-msvc
   - if defined VCVARS call "%VCVARS%" %VCVARSARG%
-  - C:\Miniconda3\Library\bin\conda.bat activate
+  - conda activate
   - pip install tox
 
 test_script: