Fix application of SOLID_QUEUE_SUPERVISOR_MODE environment variable

[lovro-bikic]

The readme states that SOLID_QUEUE_SUPERVISOR_MODE environment variable can be used to control supervisor mode. However, this isn't the case currently. When I run:

SOLID_QUEUE_SUPERVISOR_MODE=async bin/jobs

Solid Queue is still started in fork mode.

This is because Cli class sets the default value for --mode option to fork:

solid_queue/lib/solid_queue/cli.rb

Line 11 in 552f7d0

class_option :mode, type: :string, default: "fork", enum: %w[ fork async ],

so when Configuration is instantiated:

solid_queue/lib/solid_queue/configuration.rb

Lines 34 to 35 in 552f7d0

	def initialize(**options)
	@options = options.with_defaults(default_options)

options will always contain { mode: "fork" }, so default_options, which reads the SOLID_QUEUE_SUPERVISOR_MODE env variable, never applies:

solid_queue/lib/solid_queue/configuration.rb

Lines 93 to 95 in 552f7d0

	def default_options
	{
	mode: ENV["SOLID_QUEUE_SUPERVISOR_MODE"] || :fork,

This PR removes the default value from Cli class so that default_options can apply.

I didn't add any tests, but let me know if (and where) I should.