bug#48552: mesa 20.2.4 is not reproducible

[Bone Baboon via Bug reports for GNU Guix <bug-guix@gnu.org>]

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When I run `guix challenge` it shows that these versions of mesa are not
reproducible.

```
/gnu/store/2lyzzw7ybsm3s2n7fgqm17n33n62dl7c-mesa-20.2.4
/gnu/store/k72zqqiysk3qcwqj917hmlbvqs8shj0x-mesa-20.2.4
```

This looks like it could also be related to bug#43849.

`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 mesa` outputs:

```
/gnu/store/9624kmpsr9mcx0qb1zvbk5hpxymgk0s4-mesa-20.2.4 contents differ:
  local hash: 0ca468jgq1sgw0llsjrmk1jjny08ndhs4634yp9m9aajr4p0g8q1
  https://ci.guix.gnu.org/nar/lzip/9624kmpsr9mcx0qb1zvbk5hpxymgk0s4-mesa-20.2.4: 1kp0l03bdinmnzkn2xqyrl6gymzy9qrh5kbq2i8fbgxb0c1j54m7
  differing files:
    /lib/dri/iris_dri.so
    /lib/dri/nouveau_drv_video.so
    /lib/vdpau/libvdpau_nouveau.so.1.0.0
    /lib/libXvMCnouveau.so.1.0.0
    /lib/libOSMesa.so.8.0.0
    /lib/libvulkan_radeon.so
    /lib/libxatracker.so.2.5.0

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

Attached is the diffoscope output.

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