do not reset PS1

**Describe the bug**
Toolbox currently overrides the shell prompt (PS1).
I feel it would be better if it just prepended ⬢ to the existing PS1 rather than overwriting the setting.

**Steps how to reproduce the behaviour**
For example for Fedora 39 the default bash prompt (now green) is:

```
petersen@localhost-live:~$
```
which on `toolbox enter` then changes to:
```
⬢[petersen@toolbox ~]$
```

**Expected behaviour**
I think it would be good if the prompt just modified to:
```
⬢petersen@toolbox:~$
```
Then the prompt would also be closer to (derived from) the default system prompt for the toolbox container OS I think.

**Actual behaviour**
```
⬢[petersen@toolbox ~]$
```

Start by reproducing the prompt change with `toolbox enter` using the Fedora 39 example in the issue. No source file or test is named, so locate the entry point that sets PS1 and verify that the existing prompt is preserved while the toolbox marker is added.

Written by the indexing model from the issue text.