Dotenv inline command substitutions incorrectly matches escaped
parentheses when parsing commands, so let's replace its regex with one
that doesn't.

This means that when you have a secrets file like:

```
SECRETS=$(kamal secrets fetch --adapter myadapter SECRET_WITH_PARENS)
SECRET_WITH_PARENS=$(kamal secrets extract SECRET_WITH_PARENS ${SECRETS})
```

Then we correctly add the secret even if it contains parentheses.

Currently if SECRET_WITH_PARENS=pass)word, then we get:

```
SECRET_WITH_PARENS=$(kamal secrets extract SECRET_WITH_PARENS pass\)word)
```

And dotenv's regex thinks the command is
`kamal secrets extract SECRET_WITH_PARENS pass\`.