Merge bitcoin/bitcoin#22465: guix: Pin kernel-header version, time-machine to upstream 1.3.0 commit

e6a94d4446 guix: Bump to version-1.3.0 from upstream (Carl Dong)
90fd13b954 guix: Pin kernel header version (Carl Dong)

Pull request description:

  ```
  - Use 4.19 for riscv64 (earliest LTS release w/ riscv64 support)
  - Use 4.9 for all others (second-oldest LTS release, released in
    combination with glibc glibc 2.24 in Debian stretch)
  ```

  ```
  The chosen commit is the HEAD of Guix's version-1.3.0 branch as of July
  15th, 2021.

  Also fix visual indenting.
  ```

  -----

  This + the documentation PR should make our Guix system ready for release!

ACKs for top commit:
  MarcoFalke:
    review ACK e6a94d4446 to change to vanilla guix. Did not review the kernel change.
  laanwj:
    ACK e6a94d4446
  fanquake:
    ACK e6a94d4446

Tree-SHA512: a175e4ddb3ee786a39f5e800ce336932ad2f6797a3a28400a6f723875d0f19833fd36cedc41b3580e4604110517211bd9f557be36adf7265fd8e591c434ae032

contrib/guix/guix-build

@@ -238,8 +238,8 @@ SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git log --format=%at -1)}"
 # across time.
 time-machine() {
     # shellcheck disable=SC2086
-    guix time-machine --url=https://github.com/dongcarl/guix.git \
-                      --commit=490e39ff303f4f6873a04bfb8253755bdae1b29c \
+    guix time-machine --url=https://git.savannah.gnu.org/git/guix.git \
+                      --commit=aa34d4d28dfe25ba47d5800d05000fb7221788c0 \
                       --cores="$JOBS" \
                       --keep-failed \
                       --fallback \