As of version 1.4.0 and later, the TortoiseSVN installer doesn't provide the user with the option to set the
SVN_ASP_DOT_NET_HACK environment variable anymore, since that caused many problems and confusions with users which always install everything no matter if they know what it is for.
But that option is only hidden for the user. You still can force the TortoiseSVN installer to set that environment variable by setting the
ASPDOTNETHACK property to TRUE. For example, you can start the installer like this:
msiexec /i TortoiseSVN-1.4.0.msi ASPDOTNETHACK=TRUE