feat(runner): give .bin binaries priority and skip their builds

[mkoura]

Binaries placed in the repo's .bin directory were always shadowed by the nix-built ones, because the built bin dirs were prepended in front of .bin in PATH_PREPEND. Reverse the order so that .bin always wins, and skip the (nix) build of any binary for which a usable executable is already present in .bin.

• Append built bin dirs after .bin in PATH_PREPEND instead of prepending, and move the db-sync setup after the node/cli setup so its bin dirs also come after theirs.
• Add is_usable_binary and report_existing_binary helpers. The latter logs the binary's version (best effort) and hard-fails binaries that cannot run at all (wrong arch, missing libs, crash on startup), since they would still shadow working ones on PATH.
• Skip builds per binary: node bundle (via new skip_bindir args), standalone cardano-cli, tx-centrifuge, cardano-db-sync and cardano-smash-server. The db-sync repo is still cloned for the schema files and the release tarball is still downloaded.
• Categorize .bin entries early and fail fast on executable but unusable entries (e.g. empty files) that would shadow built binaries as silent no-ops.
• Re-verify all required binaries on the final PATH before the testrun and exit 3 (distinct from pytest's 1) when any is missing or unusable. Create the artifacts dirs upfront so this exit code is not masked by grep_errors.sh.
• Document the .bin behavior in the README.

[Copilot]

Pull request overview

This PR updates the runner scripts to ensure user-provided executables in the repo’s .bin directory take precedence over nix-built binaries, and to skip building binaries when a usable one is already present in .bin. It also adds helper functions to validate/report existing binaries and documents the .bin behavior.

Changes:

• Add is_usable_binary, report_existing_binary, and assert_cmds_available helpers to support .bin prioritization and validation.
• Update runner/regression.sh (and supporting scripts) to (a) prefer .bin on PATH, (b) skip nix builds when .bin provides a usable binary, and (c) re-check required commands before running tests.
• Update db-sync and cardano-node setup scripts to support build skipping and adjust PATH ordering; document .bin behavior in README.md.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
scripts/common.sh	Adds helpers for determining “usable” executables, reporting existing binaries, and asserting required commands exist.
runner/source_dbsync.sh	Allows skipping db-sync/smash builds when .bin provides usable binaries; adjusts PATH handling and adds helper dependency check.
runner/source_cardano_node.sh	Adds optional skip_bindir to skip builds and omit PATH entries for binaries already present in .bin.
runner/regression.sh	Reorders PATH so .bin wins, categorizes .bin entries early, skips builds when possible, and re-verifies required commands before testrun.
README.md	Documents .bin usage and container constraints for custom binaries.

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