sh-dsl cannot unquote environment variable value

[fynzhak]

It looks like it's not possible to unquote the environment variable value when using sh-dsl.
For example:

(var bar "asdf")
($ FOO=,bar janet -e "(print (os/getenv `FOO`))")

results in a compilation error:
compile error: error: (macro) expected integer key for symbol in range [0, 0), got 0

I found this a bit surprising given that unquote works elsewhere in the expression.

Is this something we want to support?

[bakpakin]

We don't currently recognize setting environment variables with two separate tokens - FOO=BAR is parsed as one symbol, but FOO=,bar is parsed as two, FOO= and (unquote bar).

I think this could be ambiguous in some cases, e.g. ($ FOO= ,janet -e '(print :hi)'), it's not clear if FOO binds to the empty string or to ,janet

[fynzhak]

I had a look at your commit and that's pretty much what I've been playing with as a solution 😃
I rationalized the ambiguity to myself that one probably wants to set an environment variable to a value, so it's probably OK to require both the key and value to be specified. Trying to unquote the value currently results in a bad command anyway, so it's likely a mistake someone makes only once.

I've also been playing with handling an :unset keyword in case someone indeed wants to unset the environment variable entirely.
($ FOO=,:unset ...)
But maybe that's going too far?

Thanks for the change!