kpkginstall: use the right compiler for the cross compiling workaround

The build system sets various config options based on the compiler/linker used to build the kernel. These config options can affect the build process, so we need to ensure that the right compiler/linker (i.e. gcc vs. clang) is used for the cross compiling workaround.

Without this the workaroud may, for example, fail to rebuild a tool that is actually needed to build a kernel module when the correct compiler-linker combo is used, leading to test failures.

Should fix #1507 (closed)