sysstat compresses manual pages using the `ZIP` environment variable
unless `COMPRESS_MANPG` is set.
`ZIP` seems to default to xz(1) and is for more than just compressing
manuals.
At least with `documentation.man.mandoc.enable = true;` this renders
them unreadable:
```
$ man -w mpstat
man: outdated mandoc.db lacks mpstat(1) entry, run makewhatis /nix/store/lk0r6ibvkrvaq96gp8h4rqgrd2q0c8q5-system-path/share/man
/nix/store/lk0r6ibvkrvaq96gp8h4rqgrd2q0c8q5-system-path/share/man/man1/mpstat.1.xz
```
(followed by `PAGER` showing compressed mpstat.1.xz content.)
Disable manual page compression during build and thus defer it to Nix:
```
gzipping man pages under /nix/store/wl2dwl1lhs9g5w2d5dhr0pcskp78nz42-sysstat-12.4.5/share/man/
```
This also avoids potential runtime incompatibilites in sysstat
(I am not a `services.systat` user).
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