Allow $SHELL env variable to override shellCommand in runshellscript.go

Hey,

In [runshellscript.go](https://github.com/aws/amazon-ssm-agent/blob/master/agent/plugins/runscript/runshellscript.go#L31) the shellCommand is hard-coded to: `var shellCommand = "sh"`

So if I run a SSM document with bash specific commands, it will fail.

It would be great if the agent used something like `os.Getenv("SHELL")` if it was set, or falls back to `sh`

That way, users could set the `SHELL` env variable for the agent, in the same way proxies etc are configured.

### Example doc:
```
{
"schemaVersion": "2.0",
"description": "Test Shell",
"parameters": {},
"mainSteps": [
{
"action": "aws:runShellScript",
"name": "TestShell",
"inputs": {
"runCommand": [
" #!/bin/bash",
" IFS=', ' read -r -a array <<< 'item1, item2, item3'",
" for item in \"${array[@]}\"; do echo \"$item\"; done"
]
}
}
]
}
```
### Expected output:
> item1
item2
item3

### Actual output:
>----------ERROR-------
failed to run commands: exit status 2
/var/lib/amazon/ssm//document/orchestration//TestShell/_script.sh: 2:
/var/lib/amazon/ssm//document/orchestration//TestShell/_script.sh: Syntax error: redirection unexpected

### To reproduce in the console:
This works:
`/bin/bash /var/lib/amazon/ssm//document/orchestration//TestShell/_script.sh`

This fails:
`sh /var/lib/amazon/ssm//document/orchestration//TestShell/_script.sh`

Start in agent/plugins/runscript/runshellscript.go at the referenced shellCommand definition and inspect how the command is used to execute scripts. Verify the provided SSM document with bash-specific syntax, and consider the work done when a set SHELL value is honored while an unset value retains sh behavior.

Written by the indexing model from the issue text.