session on ubuntu with amazon-ssm-agent snap installed has non-writeable cwd

On a recent Ubuntu instance running `amazon-ssm-agent` version 3.3.987.0 (installed from the snapstore), the working directory on a newly created session is a readonly directory.
```
$ aws ssm start-session --target i-0e0d6b6d0d84843b2

Starting session with SessionId: tom-a6rd2j5dx36y98yggjegiu3poq
$ pwd
/var/snap/amazon-ssm-agent/9881
$
```

That is confusing for users especially when they try to call basic commands like:
```
$ echo "foo" > testfile
sh: 2: cannot create testfile: Permission denied
```

changing the default working directory to `/home/ssm-user` for a session would be good:

```
$ cd
$ pwd
/home/ssm-user
```

Start by reproducing the session with `aws ssm start-session --target ...` on Ubuntu with `amazon-ssm-agent` installed from the snapstore, then inspect the session startup path. The issue is done when a newly created session opens in `/home/ssm-user` and can create a file there instead of starting in the read-only `/var/snap/amazon-ssm-agent/...` directory.

Written by the indexing model from the issue text.