bug#49795: zchunk: zstd path

[Christopher Howard]

Hello, I have been experimenting with the new package zchunk. zchunk
comes with a utility zck_gen_zdict, which utilizes the zstd program.
zstd is a propogated input of zck_gen_zdict, but it appears that
zck_gen_zdict hardcodes zstd to /usr/bin:

```
christopher@theoden ~/Scratch$ zck_gen_zdict clonezilla-live-2.6.7-28-
amd64.iso.zck 
Unable to find /usr/bin/zstd
Error generating dict
```

My system information:

```
christopher@theoden ~/Scratch$ neofetch --stdout
christopher@theoden 
------------------- 
OS: Guix System 94ece4f8f7c80133d759d302b11351b69919209c x86_64 
Host: OptiPlex 9020 00 
Kernel: 5.13.6-gnu 
Uptime: 1 hour, 28 mins 
Packages: 94 (guix-system), 107 (guix-user) 
Shell: bash 5.0.16 
Resolution: 1920x1080 
DE: GNOME 
Theme: Adwaita [GTK2/3] 
Icons: Adwaita [GTK2/3] 
Terminal: .gnome-terminal 
CPU: Intel i5-4570 (4) @ 3.600GHz 
GPU: Intel HD Graphics 
GPU: AMD ATI Radeon HD 8490 / R5 235X OEM 
Memory: 2232MiB / 7869MiB 
```

-- 
Christopher Howard
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