ci: make release publishing resumable

[pepicrft]

Summary

• Compile HexDocs with NIF stubs so release docs do not invoke Rust source builds
• Add a microsandbox NIF disable flag for compile-only tasks
• Make release publishing tolerate an existing tag or GitHub release during reruns

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/release.yml"); puts "ok"'
• git diff --check
• CONDUKT_BASHKIT_DISABLE=1 CONDUKT_MICROSANDBOX_DISABLE=1 MIX_ENV=dev mix docs

Note: mix docs still reports existing ExDoc reference warnings unrelated to this change.