`dap-debug': don't clobber envvars

If the :environment-variables key is given, all of its variables are set using
`setenv', in a way that affects the entire running Emacs instance, instead of
just the subprocess.

The solution is to let-bind `process-environment' to a copy of itself if
:environment-variables is given.

Author: nbfalcon

dap-mode.el

@@ -1591,6 +1591,9 @@ before starting the debug process."
                    :startup-function :environment-variables :hostName host) launch-args)
           (session-name (dap--calculate-unique-name name (dap--get-sessions)))
           (default-directory (or cwd default-directory))
+          (process-environment (if environment-variables
+                                   (cl-copy-list process-environment)
+                                 process-environment))
           program-process)
     (mapc (-lambda ((env . value)) (setenv env value)) environment-variables)
     (plist-put launch-args :name session-name)