`humility exec` doesn't seem to like (single) quotes in commands

This is Not A Big Deal (as it can be fixed by removing the quotes), but:

eliza@tereshkova ~ $ humility -t sn17 exec --list
NAME           COMMAND
console        /bin/sh -c "sp3-uart /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_BG00RVDP-if00-port0"
power.off      humility -t sn17 hiffy -c Sequencer.set_state -a state=A2
power.on       humility -t sn17 hiffy -c Sequencer.set_state -a state=A0
power.state    humility -t sn17 hiffy -c 'Sequencer.get_state'

eliza@tereshkova ~ $ humility -t sn17 exec power.state
humility: sn17 power.state: executing: 'humility -t sn17 hiffy -c 'Sequencer.get_state'' ...
humility: attached to 0483:3754:003200124741500820383733 via ST-Link V3
humility hiffy failed: interface 'Sequencer not found

Stack backtrace:
   0: humility_idol::IdolOperation::new
   1: humility_cmd_hiffy::hiffy
   2: humility_cmd::attach
   3: humility::cmd::subcommand
   4: humility::main
   5: std::sys::backtrace::__rust_begin_short_backtrace
   6: std::rt::lang_start::{{closure}}
   7: std::rt::lang_start_internal
   8: main
   9: __libc_start_call_main
  10: __libc_start_main_alias_2
  11: _start
humility: sn17 power.state: done (status code 1)

I had gotten in the habit of always single-quoting arguments to humility hiffy commands, so I was a bit surprised that it doesn't work in the environment file.

Start with the humility exec command and the environment-file command parsing, then reproduce the shown power.state example with its single-quoted argument. Done means quoted commands from the environment file execute without stripping or duplicating the quotes, while the existing unquoted commands continue to work.