recipes: Start WORKDIR -> UNPACKDIR transition

Replace references of WORKDIR with UNPACKDIR where it makes sense to do
so in preparation for changing the default value of UNPACKDIR.

(From OE-Core rev: 1f18b9a512800860d5153d89eb82b56388efad6f)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/lua/lua_5.4.6.bb

@@ -51,7 +51,7 @@ do_install () {
 }
 
 do_install_ptest () {
-        cp -R --no-dereference --preserve=mode,links -v ${WORKDIR}/lua-${PV_testsuites}-tests ${D}${PTEST_PATH}/test
+        cp -R --no-dereference --preserve=mode,links -v ${UNPACKDIR}/lua-${PV_testsuites}-tests ${D}${PTEST_PATH}/test
 }
 
 do_install_ptest:append:libc-musl () {