build: Move guix time machine to prelude

This deduplicates some code, and enforces consistency of the time
machine configuration between scripts.

contrib/guix/libexec/prelude.bash

@@ -45,6 +45,22 @@ EOF
 exit 1
 fi
 
+################
+# Execute "$@" in a pinned, possibly older version of Guix, for reproducibility
+# across time.
+time-machine() {
+    # shellcheck disable=SC2086
+    guix time-machine --url=https://git.savannah.gnu.org/git/guix.git \
+                      --commit=ae03f401381e956c4c41b4cf495cbde964fa43d0 \
+                      --cores="$JOBS" \
+                      --keep-failed \
+                      --fallback \
+                      ${SUBSTITUTE_URLS:+--substitute-urls="$SUBSTITUTE_URLS"} \
+                      ${ADDITIONAL_GUIX_COMMON_FLAGS} ${ADDITIONAL_GUIX_TIMEMACHINE_FLAGS} \
+                      -- "$@"
+}
+
+
 ################
 # Set common variables
 ################