Visual Studio install found through --with-tools-dir value is discarded

In toolchain_windows.m4, the --with-tools-dir configuration setting is used by TOOLCHAIN_CHECK_POSSIBLE_VISUAL_STUDIO_ROOT to find a valid Visual Studio installation, and the VS_ENV_CMD variable is set. However, after calling this macro, the VS_ENV_CMD is set to an empty string again, effectively discarding the result, and falling back to other methods for finding a valid Visual Studio installation, rendering the use of --with-tools-dir effectively useless.

The fix is to set VS_ENV_CMD to an empty string before trying to use the --with-tools-dir value instead, so that any found value for it is not inadvertently discarded.