test: fix ccache

For some reason CCACHE_HASHDIR needs to be unset for ccache to work when
building the integration tests from the kpatch-test script.

test/integration/kpatch-test

@@ -45,6 +45,7 @@ ROOTDIR="$(readlink -f $SCRIPTDIR/../..)"
 # TODO: option to use system-installed binaries instead
 KPATCH="sudo $ROOTDIR/kpatch/kpatch"
 RMMOD="sudo rmmod"
+unset CCACHE_HASHDIR
 KPATCHBUILD="$ROOTDIR"/kpatch-build/kpatch-build
 ERROR=0
 LOG=test.log