Replace use of NVCC_CCBIN

[acxz]

Upstream uses:

  export GCC_HOST_COMPILER_PATH="${NVCC_CCBIN/++/cc}"
  export HOST_C_COMPILER="${NVCC_CCBIN/++/cc}"
  export HOST_CXX_COMPILER="$NVCC_CCBIN"

https://gitlab.archlinux.org/archlinux/packaging/packages/tensorflow/-/blob/main/PKGBUILD?ref_type=heads#L116-118

While we have:

  export GCC_HOST_COMPILER_PATH=/usr/bin/gcc
  export HOST_C_COMPILER=/usr/bin/${CC}
  export HOST_CXX_COMPILER=/usr/bin/${CXX}

as we do not use NVCC_CCBIN.