per-machine appveyor os config

appveyor.yml

 version: "{build}"
 
-os: Visual Studio 2019
-
 shallow_clone: true
 
 environment:
   matrix:
     # Visual Studio 2019 64bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat
+      os: Visual Studio 2019
 
     # Visual Studio 2019 32bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars32.bat
+      os: Visual Studio 2019
       machine: x86
 
     # Visual Studio 2017 64bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat
+      os: Visual Studio 2017
 
     # Visual Studio 2017 32bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat
+      os: Visual Studio 2017
       machine: x86
 
     # Visual Studio 2015 64bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat
+      os: Visual Studio 2015
       machine: amd64
 
     # Visual Studio 2015 32bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat
+      os: Visual Studio 2015
       machine: x86
 
     # Visual Studio 2013 64bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat
+      os: Visual Studio 2013
       machine: amd64
 
+    # Visual Studio 2013 32bit
+    - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat
+      os: Visual Studio 2013
+      machine: x86
 init:
   - call "%visualcpp%" %machine%
   # For using Rubyins4aller's Ruby 2.6 64bit