cmd-build: move --write-composejson-to switch to cmdlib

Now that the variable is declared publicly, we can just add it to the
list of rpm-ostree args as part of the compose helper.

This has no functional change in the `cosa build` path. In the
`cosa fetch` path, this will add the switch but rpm-ostree will ignore
it since we're not actually writing a commit.

(cherry picked from commit 4be28b6)

src/cmd-build

@@ -291,7 +291,6 @@ if test -n "${previous_commit}"; then
 fi
 RUNVM_NONET=1 runcompose_tree --cache-only ${FORCE} \
            --add-metadata-from-json "${commitmeta_input_json}" \
-           --write-composejson-to "${composejson}" \
            --ex-write-lockfile-to "${lockfile_out}".tmp \
            "${extra_compose_args[@]}"
 strip_out_lockfile_digests "$lockfile_out".tmp

src/cmdlib.sh

@@ -433,6 +433,7 @@ runcompose_tree() {
     rm -f "${changed_stamp}"
     # shellcheck disable=SC2086
     set - ${COSA_RPMOSTREE_GDB:-} rpm-ostree compose tree --repo="${repo}" \
+            --write-composejson-to "${composejson}" \
             --touch-if-changed "${changed_stamp}" --cachedir="${workdir}"/cache \
             ${COSA_RPMOSTREE_ARGS:-} --unified-core "${manifest}" "$@"