Merge bitcoin/bitcoin#29078: build: Bump guix time-machine to unlock riscv64 metal

fa87a20 build: Bump guix time-machine to unlock riscv64 metal (MarcoFalke)

Pull request description:

  Closes bitcoin/bitcoin#29020

ACKs for top commit:
  fanquake:
    ACK fa87a20

Tree-SHA512: a4681f560ebab2f74c2dc8e5a9851d23c1f3fd7235b19343950afdc126d4bc77fc733fa852f55d18656ba592f37a9fff487d8af776f6b952eb4f52c1affa8ba4

Author: fanquake

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=77386bdbfe6b0c649c05ab37f08051d1ab3e5074 \
+                      --commit=d5ca4d4fd713a9f7e17e074a1e37dda99bbb09fc \
                       --cores="$JOBS" \
                       --keep-failed \
                       --fallback \