This is another attempt at fixing the annoying nixpkgs-vet errors in CI,
which just throw with `error: SQLite database '...' is busy`.
The assumption is that this happens while initially setting up the state
directories. nixpkgs-vet runs `nix-instantiate` on both the base and the
head commit and these two could interfere.
One of the nicest perks of using scx is the ability to easily switch
between schedulers at run-time, but this is currently not possible in
NixOS, given that the scx service only uses the scheduler from the
config file.
It's possible, however, to override the scheduler and its flags using
the `SCX_SCHEDULER_OVERRIDE` and `SCX_FLAGS_OVERRIDE` environment
variables, respectively (see [more
details](https://en.opensuse.org/Pluggable_CPU_schedulers#Temporarily_switch_to_a_different_scheduler)).
So the solution here is to change the scx service executable and pass
those env vars to the service at run-time falling back to the values
from the module config as defaults.
After this change, it would be possible to override schedulers like so:
```shellSession
$ sudo systemctl set-environment SCX_SCHEDULER_OVERRIDE='scx_lavd'
$ sudo systemctl set-environment SCX_FLAGS_OVERRIDE='--performance'
$ sudo systemctl restart scx.service
```
```shellSession
$ journalctl -u scx.service -b 0 -e
Oct 18 13:09:25 nixos systemd[1]: Started SCX scheduler daemon.
Oct 18 13:09:25 nixos bash[1829848]: 13:09:25 [INFO] Performance mode is
enabled.
Oct 18 13:09:25 nixos bash[1829848]: 13:09:25 [INFO] Energy model won't
be used for CPU preference order.
Oct 18 13:09:26 nixos bash[1829848]: 13:09:26 [WARN] libbpf: map
'lavd_ops': BPF map skeleton link is uninitialized
Oct 18 13:09:26 nixos bash[1829848]: 13:09:26 [INFO] scx_lavd scheduler
is initialized (build ID: 1.0.16 x86_64-unknown-linux-gnu)
Oct 18 13:09:26 nixos bash[1829848]: 13:09:26 [INFO] scx_lavd scheduler
starts running.
```
And also restore to the module defaults:
```shellSession
$ sudo systemctl unset-environment SCX_SCHEDULER_OVERRIDE
$ sudo systemctl unset-environment SCX_FLAGS_OVERRIDE
$ sudo systemctl restart scx.service
```
```shellSession
$ journalctl -u scx.service -b 0 -e
Oct 18 13:12:56 nixos systemd[1]: Stopped SCX scheduler daemon.
Oct 18 13:12:56 nixos systemd[1]: Started SCX scheduler daemon.
Oct 18 13:12:56 nixos bash[1833220]: 13:12:56 [INFO] NUMA nodes: 1
Oct 18 13:12:56 nixos bash[1833220]: 13:12:56 [INFO] Disabling NUMA
optimizations
Oct 18 13:12:56 nixos bash[1833220]: 13:12:56 [INFO] scx_bpfland 1.0.16
x86_64-unknown-linux-gnu SMT on
```
R. Ryantm