Dynamically complete the environment variable at point.
Completes if after a variable, i.e., if it starts with a "$".
This function is similar to `comint-dynamic-complete-filename', except that it
searches `process-environment' for completion candidates. Note that this may
not be the same as the interpreter's idea of variable names. The main problem
with this type of completion is that `process-environment' is the environment
which Emacs started with. Emacs does not track changes to the environment made
by the interpreter. Perhaps it would be more accurate if this function was
called `shell-dynamic-complete-process-environment-variable'.
Returns non-nil if successful.
(fn)