guix: bump time-machine to dc4842797bfdc5f9f3f5f725bf189c2b68bd6b5a

This includes a commit to fix building LLVM 17 on riscv64, see
https://git.savannah.gnu.org/cgit/guix.git/commit/?id=4e26331a5ee87928a16888c36d51e270f0f10f90.

Followup to discussion in
bitcoin/bitcoin#28880 (comment).

If you don't have riscv64 hardware, this can be tested with the
following:
```bash
guix time-machine --commit=d5ca4d4fd713a9f7e17e074a1e37dda99bbb09fc -- build --target=riscv64-linux-gnu  llvm
....
riscv64-linux-gnu-ld: CMakeFiles/dsymutil.dir/dsymutil.cpp.o: undefined reference to symbol '__atomic_fetch_and_1@@LIBATOMIC_1.0'
riscv64-linux-gnu-ld: /gnu/store/i4ga0pnr1b74bir2bjyp8mcrrbsvk7d3-gcc-cross-riscv64-linux-gnu-11.3.0-lib/riscv64-linux-gnu/lib/libatomic.so.1:
  error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

guix time-machine --commit=dc4842797bfdc5f9f3f5f725bf189c2b68bd6b5a -- build --target=riscv64-linux-gnu  llvm
....
grafting '/gnu/store/7y0j0y8jaz4mjx2nz0y42wdnxxjp6id6-llvm-17.0.6-opt-viewer' -> '/gnu/store/8xvahrrjscbprh6cjj0qp5bm9mm78wwa-llvm-17.0.6-opt-viewer'...
grafting '/gnu/store/bjhw648bz7ijd2p9hgzzdbw1q8hpagk8-llvm-17.0.6' -> '/gnu/store/x50qi8i2ywgpx6azv4k55ms0w5xjxxg5-llvm-17.0.6'...
successfully built /gnu/store/q9xvk8gzzvb4dxfzf6yi5164zd0d1vj2-llvm-17.0.6.drv
```

contrib/guix/libexec/prelude.bash

@@ -51,7 +51,7 @@ fi
 time-machine() {
     # shellcheck disable=SC2086
     guix time-machine --url=https://git.savannah.gnu.org/git/guix.git \
-                      --commit=d5ca4d4fd713a9f7e17e074a1e37dda99bbb09fc \
+                      --commit=dc4842797bfdc5f9f3f5f725bf189c2b68bd6b5a \
                       --cores="$JOBS" \
                       --keep-failed \
                       --fallback \