ci, t, tools: use `command` and `type` instead of `which`

ci, t, tools: use `command` and `type` instead of `which`

`which` is not POSIX, and cannot be assumed to installed everywhere.

`command -v` is available in POSIX and its predecessors at least since
1994: https://pubs.opengroup.org/onlinepubs/7908799/
It can be used as a replacement for `which` in a number of occurrences
in fio.

For bash scripts, `type -P` is available as a builtin replacement for
`which` and its $PATH search semantics.

Signed-off-by: Ville Skyttä <ville.skytta@upcloud.com>