Set proxy config for daemon during installation on Darwin

[laozc]

Motivation

The macOS installer does not set the HTTP proxies for nix-daemon as like on Linux.
This may fail the downloading in certain environment.
This PR addresses the issue by setting the Launch Daemon plist file with the values read from environment variables.

Context

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[abathur]

Just connecting dots:

• would presumably fix/close: Nix installer does not configure proxies correctly for nix-daemon on MacOS #1669

• equivalent systemd behavior added in: installer: handle network proxy in systemd multi-user install #2946

• it looks like @rrbutani started some work to improve how this is handled for systemd in rrbutani@d2d2a3e (and commit message suggests they may also have/intend a commit adding a macOS equivalent on top of that work)

• a few months ago the systemd equivalent received a fix for some edge-case escaping issues (not sure, off the top of my head, if this is systemd idiomatic or would also apply to launchd plists) in: Add escape for systemd service in installer script #7598

• I saw a few comments on the downside of adopting whatever settings were active at install:

  • nix-daemon and per-user proxy settings #1472
  • Nix installer does not configure proxies correctly for nix-daemon on MacOS #1669 (comment)
  • roaming laptop: network proxy configuration nixpkgs#27535
  • installer: handle network proxy in systemd multi-user install #2946 (comment)

• I saw at least two comments ~suggesting fixes in Nix itself:

  • use libproxy: installer: handle network proxy in systemd multi-user install #2946 (comment) (already linked above)
  • add explicit config: Allow CLI to pass environment variables to daemon #5890 (comment) (the last sentence is most relevant; I take it to be suggesting something like what was already done for SSL certs in Add a setting for configuring the SSL certificates file #8062)

  Perhaps @zimbatm and/or @edolstra have thoughts?

• equivalent linux behavior is documented at

  nix/doc/manual/src/installation/env-variables.md

  Lines 52 to 61 in 7b39a38

  	## Proxy Environment Variables
  	The Nix installer has special handling for these proxy-related
  	environment variables: `http_proxy`, `https_proxy`, `ftp_proxy`,
  	`no_proxy`, `HTTP_PROXY`, `HTTPS_PROXY`, `FTP_PROXY`, `NO_PROXY`.
  	If any of these variables are set when running the Nix installer, then
  	the installer will create an override file at
  	`/etc/systemd/system/nix-daemon.service.d/override.conf` so `nix-daemon`
  	will use them.