VS: Initialize CSharp flags consistently

Use the same environment variable for the initial flags that we
use for the compiler id.

Fixes: #17250

Copyright.txt

@@ -68,6 +68,7 @@ The following individuals and institutions are among the Contributors:
 * Matthaeus G. Chajdas
 * Matthias Kretz <[email protected]>
 * Matthias Maennich <[email protected]>
+* Michael Stürmer
 * Miguel A. Figueroa-Villanueva
 * Mike Jackson
 * Mike McQuaid <[email protected]>

Modules/CMakeCSharpInformation.cmake

@@ -43,7 +43,7 @@ endif()
 # on the initial values computed in the platform/*.cmake files
 # use _INIT variables so that this only happens the first time
 # and you can set these flags in the cmake cache
-set(CMAKE_CSharp_FLAGS_INIT "$ENV{CSharpFLAGS} ${CMAKE_CSharp_FLAGS_INIT}")
+set(CMAKE_CSharp_FLAGS_INIT "$ENV{CSFLAGS} ${CMAKE_CSharp_FLAGS_INIT}")
 # avoid just having a space as the initial value for the cache
 if(CMAKE_CSharp_FLAGS_INIT STREQUAL " ")
   set(CMAKE_CSharp_FLAGS_INIT)