Windows: disable the static standard library build

We now build the experimental SDK statically and dynamically. For now,
remove the static variant build for PR testing, the release builds will
build both the static and dynamic variant. This will prevent packaging
from being run on PR testing, and fortunately, the swift toolchain
builds are going to build both already.

Author: compnerd

utils/build-windows-toolchain.bat

@@ -72,7 +72,7 @@ if not "%SKIP_PACKAGING%"=="1" set "SkipPackagingArg= "
 
 :: Build the -WindowsSDKArchitectures argument, if any, otherwise build all the SDKs.
 set "WindowsSDKArchitecturesArg= "
-if not "%WINDOWS_SDKS%"=="" set "WindowsSDKArchitecturesArg=-Windows -WindowsSDKArchitectures %WINDOWS_SDKS%"
+if not "%WINDOWS_SDKS%"=="" set "WindowsSDKArchitecturesArg=-Windows -WindowsSDKArchitectures %WINDOWS_SDKS% -WindowsSDKLinkModes dynamic"
 
 :: Build the -HostArchName argument, if any.
 set "HostArchNameArg="