Header script doesn't work when being piped or run with process substitution

### Checklist

- [X] I added a descriptive title
- [X] I searched open reports and couldn't find a duplicate

### What happened?

The script doesn't work when being piped or run with process substitution. This causes the following to fail:

```bash
curl -fsSL https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh | bash
```

And:

```bash
bash <(curl -fsSL https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh)
```

The following lines cause the issue:

https://github.com/conda/constructor/blob/84995c8316a5399bc3dc2b67f64c92be71e9eb51/constructor/header.sh#L19-L22

### Conda Info

_No response_

### Conda Config

_No response_

### Conda list

_No response_

### Additional Context

Related to https://github.com/conda/conda/issues/12477

Start with constructor/header.sh at lines 19–22 and reproduce the failure using the piped curl and process-substitution commands in the report. Trace how those lines access the script when stdin or the script path is nonstandard, then verify that both invocation forms complete successfully.

Written by the indexing model from the issue text.