Use separate ssh user for GitHub runners other than "runner" user

[enescakir]

Burak has valid reasons for it: #752 (comment)

Additionally

Users on default GitHub runners:

sphinxsearch:x:114:123:Sphinx fulltext search service,,,:/var/run/sphinxsearch:/usr/sbin/nologin
dnsmasq:x:115:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologin
mysql:x:116:125:MySQL Server,,,:/nonexistent:/bin/false
postgres:x:117:126:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash
runneradmin:x:1000:1000:Ubuntu:/home/runneradmin:/bin/bash
runner:x:1001:127:,,,:/home/runner:/bin/bash

Users on our runners:

sphinxsearch:x:114:123:Sphinx fulltext search service,,,:/var/run/sphinxsearch:/usr/sbin/nologin
dnsmasq:x:115:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologin
mysql:x:116:125:MySQL Server,,,:/nonexistent:/bin/false
postgres:x:117:126:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash
runner:x:1000:1000::/home/runner:/bin/bash

The default GitHub runners have a "runneradmin" user, similar to our proposed separate GitHub runner user. Furthermore, if we use an additional user, our "runner" user will have the same uid, 1001.

[enescakir]

Fixed in #1295