ci: propagate secrets and fix validation in S3 reusable workflows

[BKPepe]

• Added 'secrets: inherit' to reusable build workflows in kernel.yml and packages.yml to seamlessly propagate S3 credentials.
• Fixed reusable workflow validation errors by using env context instead of secrets context in step 'if' conditionals.
• Enhanced S3 support in reusable_build.yml with custom endpoints and public URLs.
• Added conditional checks in reusable_upload-file-s3.yml to skip S3 tasks if credentials are not set.

[github-actions]

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Commit 67ffc8a

• 🔶 Commit subject length: recommended max 60, required max 80 characters
• 🔶 Commit body line 1 is longer than 100 characters (is 125):
  • Added 'secrets: inherit' to reusable build workflows in kernel.yml and packages.yml to seamlessly propagate S3 credentials.
• 🔶 Commit body line 2 is longer than 100 characters (is 118):
  • Fixed reusable workflow validation errors by using env context instead of secrets context in step 'if' conditionals.
• 🔶 Commit body line 4 is longer than 100 characters (is 102):
  • Added conditional checks in reusable_upload-file-s3.yml to skip S3 tasks if credentials are not set.

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