guix: bump time-machine to 7bf1d7aeaffba15c4f680f93ae88fbef25427252

Includes:
https://git.savannah.gnu.org/cgit/guix.git/commit/?id=d428237642e1e4ac8fda4597205ffec89926c0ec.

which removes the need to build Python2, and OpenSSL 1.x when building
for Windows.

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=efc26826400762207cde9f23802cfe75a737963c \
+                      --commit=7bf1d7aeaffba15c4f680f93ae88fbef25427252 \
                       --cores="$JOBS" \
                       --keep-failed \
                       --fallback \