guix-attest: Use ascii-armor signatures

contrib/guix/guix-attest

@@ -154,7 +154,7 @@ for outdir in "${OUTDIRS[@]}"; do
             fi
         )
         echo "${outname}: Signing SHA256SUMS to produce SHA256SUMS.asc"
-        gpg --detach-sign --local-user "$gpg_key_name" --output "$outsigdir"/SHA256SUMS.asc "$outsigdir"/SHA256SUMS
+        gpg --detach-sign --local-user "$gpg_key_name" --armor --output "$outsigdir"/SHA256SUMS.asc "$outsigdir"/SHA256SUMS
         echo ""
     fi
 done