systemd unit: make EnvironmentFile optional

Previously, k3s.service was failing when the EnvironmentFile does not exist:
```
Feb 02 17:17:30 suda-ws01 systemd[1]: k3s.service: Failed to load environment files: No such file or directory
Feb 02 17:17:30 suda-ws01 systemd[1]: k3s.service: Failed to run 'start' task: No such file or directory
Feb 02 17:17:30 suda-ws01 systemd[1]: k3s.service: Failed with result 'resources'.
Feb 02 17:17:30 suda-ws01 systemd[1]: Failed to start Lightweight Kubernetes.
```

ref: https://unix.stackexchange.com/questions/404199/documentation-of-equals-minus-in-systemd-unit-files

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

k3s.service

@@ -6,7 +6,7 @@ Wants=network-online.target
 
 [Service]
 Type=notify
-EnvironmentFile=/etc/systemd/system/k3s.service.env
+EnvironmentFile=-/etc/systemd/system/k3s.service.env
 ExecStart=/usr/local/bin/k3s server
 KillMode=process
 Delegate=yes