It is relatively easy to set environment variables in LLDB, via
settings set target.env-vars DEBUG=1
However, I can't seem to find a way to append to an environment variable. The following doesn't work:
(lldb) settings set target.env-vars DEBUG=1
(lldb) settings set target.env-vars DEBUG=2$DEBUG
(lldb) settings show target.env-vars DEBUG
target.env-vars (dictionary of strings) =
DEBUG=2$DEBUG