lualoader: Support variable substitution in env var settings

We support both of the following cases of substitution:

bar="y"
foo="${bar}"
foo="$bar"

The latter substitution syntax is, of course, not recommended- all
punctuation must be considered potential variable names, and we do not go
through the effort of searching the different combinations of, for instance,
"$x.y.z" to determine if the variable is $x, $x.y, or $x.y.z.

This is not officially documented as supported, but it has worked in
forthloader for what is most likely a long time as evaluate is used to
process the right hand side of the assignment.