gitlab-ci: Use xz --threads on Debian 10

.gitlab-ci.yml

@@ -639,6 +639,8 @@ validate-x86_64-linux-deb9-dwarf:
   variables:
     TEST_ENV: "x86_64-linux-deb10"
     BIN_DIST_PREP_TAR_COMP: "./ghc-x86_64-deb10-linux.tar.xz"
+    # pxz doesn't exist; it's xz --threads
+    XZ: "xz --threads=$CORES"
   cache:
     key: linux-x86_64-deb10