ICU-20434 ICU4C AppVeyor CI builds failing on Cygwin 3.0.0. Use Cygwin to 2.11 for now.

Also remove the redundant VS2017 build from AppVeyor, since we have Azure Pipelines.

.appveyor.yml

@@ -4,32 +4,43 @@ image: Visual Studio 2017
 platform: x64
 
 # Don't clone the entire repo.
-clone_depth: 3
+clone_depth: 1
 
 # Cache things between builds to speed things up and save bandwidth.
 cache:
   - c:\icu-ci-cache
 
+# Notes:
+#  - The VS2017 build is disabled (commented out) below, as we build both VS2015 and VS2017
+#    on Azure Pipelines instead, since that is much faster.
+#
+#  - For the Cygwin build, the CYG_MIRROR URL below is from the "Cygwin Time Machine" archive,
+#    and that URL maps to the 64-bit version of Cygwin 2.11.2-1 (with timestamp 1550062412).
+#    This was the last release of Cygwin 2.11 before version 3.0.0 was released.
+#    See: http://www.crouchingtigerhiddenfruitbat.org/Cygwin/timemachine.html
+#
+#    Note: The archived Cygwin repo does not keep or retain any file signature files, so we
+#    must use the "-X" or "--no-verify" option below in the setup command.
+
 environment:
   global:
     ICU_CI_CACHE: c:\icu-ci-cache
     CYG_URL: https://cygwin.com/setup-x86_64.exe
-    CYG_MIRROR: http://cygwin.mirror.constant.com
+    CYG_MIRROR: http://ctm.crouchingtigerhiddenfruitbat.org/pub/cygwin/circa/64bit/2019/02/13/045332
     CYG_PACKAGES: automake,gcc-core,gcc-g++,make,pkg-config,perl,python3
     CYG_ROOT: c:\cygwin-root
     CYG_CACHE: '%ICU_CI_CACHE%\cygwin64'
     CYG_CACHED_SETUP: '%CYG_CACHE%\setup.exe'
 
   matrix:
-    - BUILDSYSTEM: VS2017
-      configuration: Release
-      APPVEYOR_CACHE_SKIP_RESTORE: true
-      APPVEYOR_CACHE_SKIP_SAVE: true
-
     - BUILDSYSTEM: cygwin64-gcc
       CC: gcc
       CXX: g++
       APPVEYOR_SAVE_CACHE_ON_ERROR: true
+#    - BUILDSYSTEM: VS2017
+#      configuration: Release
+#      APPVEYOR_CACHE_SKIP_RESTORE: true
+#      APPVEYOR_CACHE_SKIP_SAVE: true
 
 for:
   -
@@ -46,7 +57,7 @@ for:
             Invoke-WebRequest $env:CYG_URL -OutFile $env:CYG_CACHED_SETUP
           }
       - cmd: >-
-          %CYG_CACHED_SETUP% --quiet-mode --no-shortcuts --no-startmenu --no-desktop --upgrade-also --only-site --site "%CYG_MIRROR%" --root "%CYG_ROOT%"  --local-package-dir "%CYG_CACHE%" --packages "%CYG_PACKAGES%"
+          %CYG_CACHED_SETUP% --no-verify --quiet-mode --no-shortcuts --no-startmenu --no-desktop --upgrade-also --only-site --site "%CYG_MIRROR%" --root "%CYG_ROOT%"  --local-package-dir "%CYG_CACHE%" --packages "%CYG_PACKAGES%"
 
     # Check that we have a working Cygwin environment before building.
     before_build:
@@ -56,13 +67,13 @@ for:
     build_script:
       - '%CYG_ROOT%\\bin\\bash -lc "cd $(cygpath ${APPVEYOR_BUILD_FOLDER}) && cd icu4c/source && ./runConfigureICU Cygwin && make check"'
 
-  -
-    matrix:
-      only:
-        - BUILDSYSTEM: VS2017
-
-    build:
-      project: icu4c/source/allinone/allinone.sln
-
-    test_script:
-      - icu4c/source/allinone/icucheck.bat x64 Release
+#  -
+#    matrix:
+#      only:
+#        - BUILDSYSTEM: VS2017
+#
+#    build:
+#      project: icu4c/source/allinone/allinone.sln
+#
+#    test_script:
+#      - icu4c/source/allinone/icucheck.bat x64 Release