Call env setup script to work around missing msvcrt.lib errors with clang-win.

appveyor.yml

@@ -33,6 +33,7 @@ environment:
     - TOOLSET: clang-win
       ADDRMD: 64
       CXXSTD: 14,17
+      ENV_SCRIPT: C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
     - TOOLSET: gcc
       CXXSTD: 03,11,14,1z
@@ -89,6 +90,7 @@ build: off
 
 test_script:
   - PATH=%ADDPATH%%PATH%
+  = if not "%ENV_SCRIPT%" == "" call "%ENV_SCRIPT%"
   - if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
   - if not "%ADDRMD%" == "" set ADDRMD=address-model=%ADDRMD%
   - b2 -j %NUMBER_OF_PROCESSORS% libs/utility/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release