env-collector: custom-written "Required" field is overwritten when other env is added

[bragov4ik]

Describe the bug

if you write custom description to "Required" field the variable is checked fine (pass). However, when adding a new env variable, the table update overwrites the custom message

To Reproduce

initial table

| Variable | Req​uir​ed | Description | Default value |
| --- | --- | --- | --- |
| `OVERWRITTEN` | Custom message | | `""` |

Let's say new env NEW_ENV is added.
Generating table should output smth like:

Found missing variables:
  NEW_ENV
Ready to update markdown file: README.md
Press any key to continue...

All variables are documented

Observed behavior

Should be smth like

| Variable | Req​uir​ed | Description | Default value |
| --- | --- | --- | --- |
| `OVERWRITTEN` | | | `""` |
| `NEW_ENV` | | | `""` |

Expected behavior

| Variable | Req​uir​ed | Description | Default value |
| --- | --- | --- | --- |
| `OVERWRITTEN` | Custom message  | | `""` |
| `NEW_ENV` | | | `""` |

Additional context

I observed it in stats service; env STATS__BLOCKSCOUT_API_URL; but made up the simple example for this issue (that I didn't check) but it should work.