CROSS_TOOLCHAIN is used to build src with a different toolchain than
the bundled one. Ports also has a CROSS_TOOLCHAIN option, but it has
a different meaning. When building ports-mgmt/pkg from ports for the
package-pkg target, unset CROSS_TOOLCHAIN to prevent ports from being
confused.

This fixes 'make CROSS_TOOLCHAIN=llvm19 package-pkg' for the native
target, but cross-building (e.g., targetting powerpc from amd64) is
still broken due to an issue in pkg itself.

MFC after: 3 seconds