docs: fix typos and add GitHub Actions usage example to README

[FanatiooN]

Description

Documentation-only changes to README.md:

• Fix typos: enviroments → environments, Coniguration → Configuration,
  cachprog → cacheprog (3 occurrences), coexistance → coexistence,
  and one grammar fix (compiling may it will generate → compiling it will generate).
• Add a GitHub Actions usage example under ## Usage. The existing examples
  cover GitLab and Dockerfile RUN, but not GitHub Actions, which is a common
  setup. The example uses direct mode + S3 and pulls the prebuilt binary from
  ghcr.io/platacard/cacheprog (no go install), matching the recommendation
  already in the Installation section. It also sets cache: false on
  setup-go so GOCACHE isn't archived twice.

No source, workflow, or config changes.

TODOs

Read the Contribution guidelines.

• Generate the docs. — N/A, this PR is the docs change.
• Run the relevant tests successfully. — N/A, no code changes.
• Include release notes. If this PR is backward incompatible, include a migration guide. — N/A, docs only, not backward-incompatible.
• Update the CHANGELOG.md file: add a feature / bug fix entry for this PR under Unreleased section. — N/A, docs changes are excluded from release notes.

Release Notes (draft)

Docs: fixed typos in README and added a GitHub Actions usage example.

Migration Guide

N/A — documentation-only change, no behavior or API affected.

[xakep666]

@FanatiooN thanks for contribution.

I have a question: did you try to run github actions workflow provided in example? Or even measure: is it faster than caching provided by setup-go action.

Also please reabase to latest main.