Fix run in external terminal not propagating environment variables to program

[sewbacca]

This PR fixes #1317
and correctly passes environment variables to the program

[sewbacca]

Rereading the code, I do not handle null env unset requests. I suppose this would also affect integrated terminals, but it would need consideration of the consequences having vim.NILs flying around:

nvim-dap/lua/dap/session.lua

Lines 74 to 76 in b8e54ac

	local function json_decode(payload)
	return vim.json.decode(payload, { luanil = { object = true }})
	end

[mfussenegger]

Rereading the code, I do not handle null env unset requests. I suppose this would also affect integrated terminals, but it would need consideration of the consequences having vim.NILs flying around:

nvim-dap/lua/dap/session.lua

Lines 74 to 76 in b8e54ac

	local function json_decode(payload)
	return vim.json.decode(payload, { luanil = { object = true }})
	end

I think that's okay for now. That can be fixed later if it ever causes an actual problem with an adapter making use of it.