feat: add rancher-system-agent env.conf

[FrankYang0529]

Problem:
Run journalctl -u rancher-system-agent.service in harvester node and we can see log like:

Dec 01 02:27:39 harvester-node-0 rancher-system-agent[15569]: time="2023-12-01T02:27:39Z" level=info msg="[Applyinator] Running command: sh [-c rke2 etcd-snapshot list --etcd-s3=false 2>/dev/null]"
Dec 01 02:27:39 harvester-node-0 rancher-system-agent[15569]: time="2023-12-01T02:27:39Z" level=info msg="[Applyinator] Command sh [-c rke2 etcd-snapshot list --etcd-s3=false 2>/dev/null] finished with err: <nil> and exit code: 127"

Solution:
Add /opt/rke2/bin to rancher-system-agent PATH.

Related Issue:
harvester/harvester#4803

Test plan:

1. Install harvester.
2. Run journalctl -u rancher-system-agent.service and see rke2 etcd-snapshot list with exit code 0:

msg="[Applyinator] Running command: sh [-c rke2 etcd-snapshot list --etcd-s3=false 2>/dev/null]"
msg="[9fbfd1d387d3a1762da058468cc7c49cb89e6481208ee1bd164b864d40917b55_0:stdout]: Name                                      Location                                                                                   Size     Created"
msg="[9fbfd1d387d3a1762da058468cc7c49cb89e6481208ee1bd164b864d40917b55_0:stdout]: etcd-snapshot-harvester-node-0-1701345604 file:///var/lib/rancher/rke2/server/db/snapshots/etcd-snapshot-harvester-node-0-1701345604 56201248 2023-11-30T12:00:04Z"
msg="[9fbfd1d387d3a1762da058468cc7c49cb89e6481208ee1bd164b864d40917b55_0:stdout]: etcd-snapshot-harvester-node-0-1701388801 file:///var/lib/rancher/rke2/server/db/snapshots/etcd-snapshot-harvester-node-0-1701388801 56201248 2023-12-01T00:00:01Z"
msg="[Applyinator] Command sh [-c rke2 etcd-snapshot list --etcd-s3=false 2>/dev/null] finished with err: <nil> and exit code: 0"
msg="[K8s] updated plan secret fleet-local/custom-92281fa60bd5-machine-plan with feedback"

[Vicente-Cheng]

LGTM, thanks for fixing that!

[bk201]

LGTM!

[bk201]

@Mergifyio backport v1.2

[mergify]

backport v1.2

✅ Backports have been created

• #617 feat: add rancher-system-agent env.conf (backport #607) has been created for branch v1.2