install.sh: create user/group correctly on redhat variants
install.sh: create user/group correctly on redhat variants

Seems like adduser in redhat variants and deiban variants are incompatible,
and there is no addgroup in redhat variants.
Since adduser in install.sh is implemented on debian variants, does not work on redhat compatible.

To fix this we need to use 'useradd' / 'groupadd' instead.

Fixes #6018

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla