[Bug]: handle backtick in variable interpolation

### What version or versions you have tested?

1.2.3

### Which operating systems have you used?

- [ ] macOS
- [ ] Windows
- [x] Linux

### What did you expect to happen?

I would like the possibility to use backticks in variables which I interpolate with mlp

### What happened instead?

I have a variable with a backtick that is not correctly handled in the interpolation, it leads to a MalformedYAMLError

### Code of Conduct

- [x] I agree to follow this project’s Code of Conduct

Start by locating mlp's variable interpolation path and reproduce the issue with a variable containing a backtick on Linux using version 1.2.3. Trace why interpolation leads to MalformedYAMLError, then verify that backticks are accepted without that error.

Written by the indexing model from the issue text.