bug#39272: `man -H` doesn't use absolute path to groff

[pkill9]

when running `man -H curl`, I get the following output:

```
man: command exited with status 255: (cd /tmp/hmanCnZGIK && 
/gnu/store/l9j6dsfs2i4spfkia492wnighplvhb1c-man-db-2.9.0/libexec/man-db/zsoelim)
 | (cd /tmp/hmanCnZGIK && 
/gnu/store/l9j6dsfs2i4spfkia492wnighplvhb1c-man-db-2.9.0/libexec/man-db/manconv 
-f UTF-8:ISO-8859-1 -t UTF-8//IGNORE) | (cd /tmp/hmanCnZGIK && 
/gnu/store/5sd2yanrfv9pq8mvnf4c6pga11r6x7qh-groff-minimal-1.22.4/bin/preconv -e 
UTF-8) | (cd /tmp/hmanCnZGIK && 
/gnu/store/5sd2yanrfv9pq8mvnf4c6pga11r6x7qh-groff-minimal-1.22.4/bin/tbl) | (cd 
/tmp/hmanCnZGIK && groff -mandoc -Thtml)
```

When I go into a guix environment containing groff however, it works.
Looking at the command `man -H` tries to use, it needs an absolute path
to groff.