Make SSHMaster::startCommand work on an args list

[Ericson2314]

Motivation

This avoids split-on-whitespace errors:

• No more bash -c needed

• No more shellEscape needed

• remote-program ssh store setting also cleanly supports args (e.g. nix daemon)

• ssh uses -- to separate args for SSH from args for the command to run.

and will help with Hydra dedup: NixOS/hydra#1339

Context

Some code taken from #6628.

Priorities and Process

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[thufschmitt]

LGTM, I'm just a bit concerned about it accidentally breaking the semantics of remote-program in some cases (because we're now the ones doing the word splitting instead of the shell). Does something like remote-program = some\ thing\ with\ escaped" and quoted spaces" behaves the same?

It's probably not the end of the world if it doesn't, but we should probably

1. Be aware of it, and mention it in the release notes if we break that
2. Provide a way for people to write arguments containing spaces in the setting

[Ericson2314]

$ NIX_CONFIG='substituters = some\ thing\ with\ escaped" and quoted spaces"' nix show-config --extra-experimental-features nix-command --json | jq .substituters.value
[
  "some\\",
  "thing\\",
  "with\\",
  "escaped\"",
  "and",
  "quoted",
  "spaces\""
]

afraid the current setting thing doesn't handle this well at all.