bug#48534: gtk+ 3.24.24 is not reproducible

[Bone Baboon]

`guix challenge` shows that these versions of gtk+ are not
reproducible.

```
/gnu/store/5y1z0cl6g8ds86xhn2h9n1pk2hrjhh1h-gtk+-3.24.24
/gnu/store/8057xmgbnb8qkb1fn6jw3xcjx81fs92h-gtk+-3.24.24
/gnu/store/f354nhi011n2sz5vh266h8ss3rhadi1g-gtk+-2.24.32
```

`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 gtk+` outputs:

```
/gnu/store/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk+-3.24.24 contents differ:
  local hash: 0g3h3d6djx3jhmgg2aawqdxj3hpq0ciyv64drhikflrpqqbmrrwh
  
https://ci.guix.gnu.org/nar/lzip/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk%2B-3.24.24:
 09963h5b7j19xq3rl1p3kk85shj9yvknlagp0jwfy9saj88m20kn
  differing file:
    /share/icons/hicolor/icon-theme.cache

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

`guix challenge --diff=diffoscope gtk+` outputs:

```
/gnu/store/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk+-3.24.24 contents differ:
  local hash: 0g3h3d6djx3jhmgg2aawqdxj3hpq0ciyv64drhikflrpqqbmrrwh
  
https://ci.guix.gnu.org/nar/lzip/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk%2B-3.24.24:
 09963h5b7j19xq3rl1p3kk85shj9yvknlagp0jwfy9saj88m20kn
--- /tmp/guix-directory.0OZAet
+++ /gnu/store/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk+-3.24.24
│   --- /tmp/guix-directory.0OZAet/share
├── +++ /gnu/store/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk+-3.24.24/share
│ │   --- /tmp/guix-directory.0OZAet/share/icons
│ ├── +++ /gnu/store/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk+-3.24.24/share/icons
│ │ │   --- /tmp/guix-directory.0OZAet/share/icons/hicolor
│ │ ├── +++ 
/gnu/store/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk+-3.24.24/share/icons/hicolor
│ │ │ │   --- /tmp/guix-directory.0OZAet/share/icons/hicolor/icon-theme.cache
│ │ │ ├── +++ 
/gnu/store/ayk8rab96lw7kx69rvfkhd6dp9p2v9ny-gtk+-3.24.24/share/icons/hicolor/icon-theme.cache
│ │ │ │┄ xxd not available in path. Falling back to Python hexlify.
│ │ │ │ @@ -7,11 +7,11 @@
│ │ │ │  0005000400000000000400040000000000030004000000000002000400000000
│ │ │ │  00010004000000000000000400000000ffffffff000000fc0000011067746b33
│ │ │ │  2d7769646765742d666163746f72790000000006000500040000000000040004
│ │ │ │  0000000000030004000000000002000400000000000100040000000000000004
│ │ │ │  00000000ffffffff000001500000016c67746b332d64656d6f2d73796d626f6c
│ │ │ │  69632e73796d626f6c6963000000000600050004000000000004000400000000
│ │ │ │  0003000400000000000200040000000000010004000000000000000400000000
│ │ │ │ -00000006000001bc000001cc000001d8000001e4000001f0000001fc32353678       
│ │ │ │ -3235362f617070730000000031367831362f61707073000032347832342f6170       
│ │ │ │ -7073000033327833322f61707073000034387834382f61707073000032327832       
│ │ │ │ +00000006000001bc000001c8000001d8000001e4000001f0000001fc33327833       
│ │ │ │ +322f617070730000323536783235362f617070730000000031367831362f6170       
│ │ │ │ +7073000034387834382f61707073000032347832342f61707073000032327832       
│ │ │ │  322f617070730000

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