#588 Pin all GitHub Actions to SHA

[VasilevNStas]

Problem

All 10 workflow files use mutable tags (@v6, @v1, @master, etc.) for GitHub Actions. A tag can be reassigned to a different commit, creating a supply-chain security risk.

Solution

Pinned every action reference to its current commit SHA:

File	Action	SHA
actionlint.yml	actions/checkout	df4cb1c0
actionlint.yml	reviewdog/action-actionlint	be0761e4
codecov.yml	actions/checkout	df4cb1c0
codecov.yml	ruby/setup-ruby	d45b1a4e
codecov.yml	codecov/codecov-action	a99c28d3
copyrights.yml	actions/checkout	df4cb1c0
copyrights.yml	yegor256/copyrights-action	b4c048dc
markdown-lint.yml	actions/checkout	df4cb1c0
markdown-lint.yml	DavidAnson/markdownlint-cli2-action	b847b0e2
pdd.yml	actions/checkout	df4cb1c0
pdd.yml	volodya-lombrozo/pdd-action	69842b56
rake.yml	actions/checkout	df4cb1c0
rake.yml	ruby/setup-ruby	d45b1a4e
reuse.yml	actions/checkout	df4cb1c0
reuse.yml	fsfe/reuse-action	772649cf
typos.yml	actions/checkout	df4cb1c0
typos.yml	crate-ci/typos	dbe52f72
xcop.yml	actions/checkout	df4cb1c0
xcop.yml	g4s8/xcop-action	4e93f123
yamllint.yml	actions/checkout	df4cb1c0
yamllint.yml	ibiqlik/action-yamllint	ae1abb28

Closes #588

[VasilevNStas]

@yegor256 plz review this PR