sysctl options declared in config.json not applied to container

### Description

It seems that, when starting a container via `runsc run` from an OCI specification file, sysctl options in the specification file won't take effect inside the container.

### Steps to reproduce

Make a rootfs directory and unpack a debian-slim image into it:
`mkdir bundle && cd bundle && mkdir --mode=0755 rootfs`
`docker export $(docker create debian:bullseye-slim) | sudo tar -xf - -C rootfs --same-owner --same-permissions`

Create a simple script in rootfs to dump a sysctl setting:
```
$ cat rootfs/test_script
#!/bin/bash

cat /proc/sys/net/ipv4/tcp_keepalive_time
```

Generate a config.json: `runsc spec -- /test_script`

Add the following to the `"linux"` block of the config:
```
"sysctl":{
"net.ipv4.tcp_keep_alive_time": "1000"
}
```

Then `sudo runsc run test_script` will show a value of 7200, not 1000.

### runsc version

```shell
runsc version 40a09da5a1ab
spec: 1.1.0-rc.1
```

### docker version (if using docker)

_No response_

### uname

Linux 5.15.0-101.103.2.1.el9uek.x86_64 #2 SMP Tue May 2 01:10:45 PDT 2023 x86_64 x86_64 x86_64 GNU/Linux

### kubectl (if using Kubernetes)

_No response_

### repo state (if built from source)

_No response_

### runsc debug logs (if available)

_No response_