bug#24028: Linux-libre kernel does not build deterministically

[Bone Baboon]

Linux-libre is not reproducible.

`guix describe` outputs:

```
Generation 24   May 12 2021 18:06:24    (current)
  guix d6aeebb
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: d6aeebb23639258311fdfb9dbf5f903079fde51a
```

`guix challenge
/gnu/store/k029d836d3slh3s5k20306zvnph5ggq4-linux-libre-5.11.16`
outputs:

```
/gnu/store/k029d836d3slh3s5k20306zvnph5ggq4-linux-libre-5.11.16 contents differ:
  local hash: 1xkzhdkqbnh1prryz2bbbz1fsydsb9srhzpj27mpx1dv47fq2r05
  
https://ci.guix.gnu.org/nar/zstd/k029d836d3slh3s5k20306zvnph5ggq4-linux-libre-5.11.16:
 0w8n65xr91g9kiwik8kf8h1jn1scwfjcbwwnqbrfiyv8k7gr8kql
  differing file:
    /bzImage

1 store items were analyzed:
  - 0 (0.0%) were identical
  - 1 (100.0%) differed
  - 0 (0.0%) were inconclusive
```

`guix challenge
/gnu/store/rcav1xxn4ajkbnkmrnl6jr5d1zb20kq1-linux-libre-5.11.14`
outputs:

```
/gnu/store/rcav1xxn4ajkbnkmrnl6jr5d1zb20kq1-linux-libre-5.11.14 contents differ:
  local hash: 1z2v884l98ysl0vass4dchwfpavnk47x8brcx8h31qp3bj1xyhaz
  
https://ci.guix.gnu.org/nar/zstd/rcav1xxn4ajkbnkmrnl6jr5d1zb20kq1-linux-libre-5.11.14:
 1nlmjamm0yc2mjwdy0rq7qplilrwz1b8zm4m688kra21xqx09aar
differing file:
    /bzImage

1 store items were analyzed:
  - 0 (0.0%) were identical
  - 1 (100.0%) differed
  - 0 (0.0%) were inconclusive
```

Attached is the diffoscope output.

diffoscope-linux-libre-5.11.14.txt.lz
Description: Binary data

diffoscope-linux-libre-5.11.16.txt.lz
Description: Binary data