qenv.error: cannot add bindings to a locked environment when evaluating qenv code:

### What happened?

When creating an S4 class inside `eval_code` you get an error that you `cannot add bindings to a locked environment`

```r
library(teal.data)
library(teal.code)
code <- c(
"setClass('aclass', slots = c(a = 'numeric', x = 'numeric', y = 'numeric')) # @linksto a x",
"x <- new('aclass', a = 1:3, x = 1:3, y = 1:3)",
"a <- new('aclass', a = 1:3, x = 1:3, y = 1:3)",
"a@x <- a@y",
"a@x <- a@x + 2",
"a@x <- x@a"
)
tdata <- eval_code(teal_data(), code)
tdata

```

```r

```

Start at the eval_code() entry point and run the reproducible example from the issue, including the setClass() call and subsequent S4 assignments. Trace how the evaluation environment is created or locked; done means the example evaluates without the qenv.error about adding bindings to a locked environment.

Scritto dal modello di indicizzazione a partire dal testo della issue.