ci: Disable pytest-xdist on cygwin

Fixes breakage when installing psutil:

Collecting psutil>=3.0.0
  Downloading psutil-5.7.2.tar.gz (460 kB)
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/cygdrive/c/Users/VSSADM~1/AppData/Local/Temp/pip-install-dajoe3xd/psutil/setup.py'"'"'; __file__='"'"'/cygdrive/c/Users/VSSADM~1/AppData/Local/Temp/pip-install-dajoe3xd/psutil/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base '/cygdrive/c/Users/VSSADM~1/AppData/Local/Temp/pip-pip-egg-info-1ei2_y44'
         cwd: /cygdrive/c/Users/VSSADM~1/AppData/Local/Temp/pip-install-dajoe3xd/psutil/
    Complete output (1 lines):
    platform cygwin is not supported

azure-pipelines.yml

@@ -111,8 +111,8 @@ jobs:
       displayName: Install Dependencies
     - script: |
         set PATH=%CYGWIN_ROOT%\bin;%SYSTEMROOT%\system32
-        env.exe -- python3 -m pip --disable-pip-version-check install gcovr pefile pytest-xdist jsonschema
+        env.exe -- python3 -m pip --disable-pip-version-check install gcovr pefile jsonschema
-      displayName: pip install gcovr pefile pytest-xdist jsonschema
+      displayName: "pip install gcovr pefile jsonschema (pytest-xdist broken, skipped: CHECK ME AGAIN)"
     - script: |
         set BOOST_ROOT=
         set PATH=%CYGWIN_ROOT%\bin;%SYSTEMROOT%\system32