guix: Rebase on 95aca2991b (1.2.0-12.dffc918)

A few changes which are useful for us:

1. 'gnu: cross-gcc-arguments: Enable 128 bit long double for POWER9.' is
   now merged into master.
2. gnutls is bumped to 3.6.15 and the temporal test failure in
   status-request-revoked is fixed. Note that this does not fix the case
   where one has installed Guix v1.2.0 and is running a substitute-less
   bootstrap build, since the `guix time-machine` command itself has a
   dependency on gnutls v3.6.12 (the one with the broken test) and will
   thus try to build it before attempting to jump forwards in time. This
   does however, mean that those who build a version of Guix that also
   contains this fix will not go backwards in time to build the broken
   gnutls v3.6.12.

contrib/guix/guix-build.sh

@@ -163,7 +163,7 @@ SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git log --format=%at -1)}"
 time-machine() {
     # shellcheck disable=SC2086
     guix time-machine --url=https://github.com/dongcarl/guix.git \
-                      --commit=6c9d16db962a6f7155571b36eced681fd2889e23 \
+                      --commit=490e39ff303f4f6873a04bfb8253755bdae1b29c \
                       --max-jobs="$MAX_JOBS" \
                       --keep-failed \
                       ${SUBSTITUTE_URLS:+--substitute-urls="$SUBSTITUTE_URLS"} \