task env declarations

[mlin]

We already implemented this feature, but we need to check it in detail against the spec language, particularly around escaping.

[mlin]

Spec clarification probably needed here:

When a value is declared with the env modifier, it becomes the execution engine's responsibility to escape the string thus preventing any sort of interpolation. Functionally, using an environment variable is the same as first escaping the string of any special characters and then wrapping it single quotes.

single_quote(escape(${variable}))

It seems ambiguous between (i) the shell interpreter's escaping syntax used to set an environment variable, and (ii) the value actually stored in the environment variable. The latter should (?) not be escaped.