Update runner to replace set-output

[SirNate0]

Github is deprecating set-output in their runners: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Starting 1st June 2023 workflows using save-state or set-output commands via stdout will fail with an error.

We use this in checking the source checksum in the build runner

U3D/.github/workflows/linux.yml

Lines 44 to 46 in 3d0d370

	- name: Source checksum
	id: source_checksum
	run: rake source_checksum

U3D/rakefile

Lines 338 to 344 in 3d0d370

	task :source_checksum do
	require 'digest'
	sha256_final = Digest::SHA256.new
	sha256_iter = Digest::SHA256
	Dir['Source/**/*.{c,h}*'].each { |it| sha256_final << sha256_iter.file(it).hexdigest }
	puts "::set-output name=hexdigest::#{sha256_final.hexdigest}"
	end

Presently this just produces the following warning, but the end-date is listed above.

Warning: The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

[Okkoma]

see merged PR #74 => to close