chore: skip upstream image workflow in fork

[DenisCarriere]

Summary

• run the inherited private Blockscout image publisher only in blockscout/blockscout
• allow its dependent Blockscout E2E deployment job to skip automatically in forks
• keep the Pinax Firehose image workflow as the fork publisher

Why

The inherited workflow targets a self-hosted runner labeled build and publishes to Blockscout-owned private images and deployment infrastructure. The Pinax fork has no such runner, leaving every push permanently queued even though the Pinax image workflow succeeds.

Validation

• mix format completed in the pinned Elixir 1.19.4 / OTP 27 toolchain
• workflow YAML parsed successfully
• git diff --check passed
• all PR checks passed after one retry of the inherited Explorer compiler-download mock flake
• Explorer retry: 167 doctests, 1529 tests, 0 failures (1 excluded)
• post-fix: stabilize V2 exchange rate channel test #10 expanded Blockscout matrix passed on rerun (95 jobs)
• Pinax Firehose image publishing passed

Commit: a8fb80367