helpers/envkernel.sh: disable ccache (MR 2189)

Not extensively tested, but this shouldn't be necessary given that
you get incremental builds with envkernel and may reduce build
times.

helpers/envkernel.sh

@@ -220,6 +220,7 @@ set_alias_make() {
 	# Build make command
 	cmd="echo '*** pmbootstrap envkernel.sh active for $PWD! ***';"
 	cmd="$cmd pmbootstrap -q chroot --user --"
+	cmd="$cmd CCACHE_DISABLE=1"
 	cmd="$cmd ARCH=$arch"
 	if [ "$need_cross_compiler" = 1 ]; then
 		cmd="$cmd CROSS_COMPILE=$cross_compiler"