ci: AppVeyor: only install PyPy if used [skip travis]

Uses powershell for simpler matching, adding with `--no-progress` to
silence choco-install.

appveyor.yml

@@ -15,13 +15,12 @@ init:
   - ps: echo $env:TOXENV
   - ps: ls C:\Python*
 install:
-  # install pypy using choco (redirect to a file and write to console in case
-  # choco install returns non-zero, because choco install python.pypy is too
-  # noisy)
-  - choco install python.pypy > pypy-inst.log 2>&1 || (type pypy-inst.log & exit /b 1)
-  - set PATH=C:\tools\pypy\pypy;%PATH% # so tox can find pypy
-  - echo PyPy installed
-  - pypy --version
+  - ps: |
+      if ($env:TOXENV -notmatch 'pypy') {
+        choco install --no-progress python.pypy
+        $env:PATH = "C:\tools\pypy\pypy;$env:PATH"
+        pypy --version
+      }
 
   # Upgrade virtualenv for e.g. more-itertools to be handled properly.
   # Pin it to work around https://github.com/tox-dev/tox/issues/1389.