fix(shellcheck-run-steps): pass all pipeline dirs via --pipeline-dirs

[dannf]

The problem

Commit 399c363 (#36) tried to let the hook find pipelines that live outside a package's own directory. It did this by passing several --pipeline-dir flags to melange compile. But melange compile declares --pipeline-dir as a single string. Repeated flags collapse to the last one. For an enterprise package, that last flag is ./pipelines/os. A package that uses a pipeline from ./pipelines/ still fails, with the error unable to load pipeline: open pipelines/test/manylinux.yaml: file does not exist. This was verified against chainguard-dev/stereo#330877 with melange v0.60.0:

--pipeline-dir=./pipelines/                      rc=0
--pipeline-dir=./enterprise-packages/pipelines/  rc=1
three flags as passed by 399c363                 rc=1

The change

chainguard-dev/melange#2656 is merged. It adds --pipeline-dirs to melange compile, the same comma-separated list flag that melange build already has. _pipeline_dirs_for now returns one --pipeline-dirs=a,b,c flag, and the fallback for unknown directories uses the same flag.

Do not merge before the image has the flag

The hook runs cgr.dev/chainguard/melange:latest. Until that image includes melange#2656, melange compile rejects --pipeline-dirs as an unknown flag, and the hook fails for every package. Check with:

docker run --rm cgr.dev/chainguard/melange:latest compile --help | grep pipeline-dirs

Evidence

This was tested with a melange binary built from #2656. Running melange compile --pipeline-dirs=./enterprise-packages/pipelines/,./pipelines/,./pipelines/os enterprise-packages/libvpx.yaml compiles the package from stereo#330877 that fails today. The function's output for each package tier:

enterprise-packages/libvpx.yaml  --pipeline-dirs=./enterprise-packages/pipelines/,./pipelines/,./pipelines/os
extra-packages/foo.yaml          --pipeline-dirs=./extra-packages/pipelines/,./pipelines/os
os/zlib.yaml                     --pipeline-dirs=./os/pipelines/
other/bar.yaml                   --pipeline-dirs=./other/pipelines

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