guix: bump time-machine to 998eda3067c7d21e0d9bb3310d2f5a14b8f1c681

There are two reasons to perform this bump:
* Fixes #25082 by bumping to a commit that includes a fix for time-dependent unit
tests in libgit2 (f5fe0082abe4547f3fb9f29d8351473cfb3a387b).
* Gives us access to clang-toolchain-14 (14.0.3, 998eda3067c7d21e0d9bb3310d2f5a14b8f1c681),
which is useful for the Guix portion of #21778.

Note that with this bump:
Linux kernels headers update from 5.15.28 to 5.15.37.

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=34e9eae68c9583acce5abc4100add3d88932a5ae \
+                      --commit=998eda3067c7d21e0d9bb3310d2f5a14b8f1c681 \
                       --cores="$JOBS" \
                       --keep-failed \
                       --fallback \