Support Debusine builds from externally-prepared source trees: srcpkg-artifact delivery and assemble-orig flag

[bjordiscollaku]

Summary

Extend the reusable workflow to support packages that do not follow the
single-repository packaging model. Two new inputs — srcpkg-artifact
and assemble-orig — unlock Debusine builds for packages like
pkg-linux-qcom where the kernel source (qualcomm-linux/kernel) and
the packaging metadata (debian/) live in separate repositories and are
injected into a single source tree at build time by prepare-source.sh.
The resulting tree has no upstream tag, no pristine-tar branch, and no
debian/gbp.conf — making the existing git-checkout and gbp-based paths
structurally inapplicable.

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Motivation

The existing workflow has a single model for source tree delivery: check
out the packaging repository at debian-ref into srcpkg/. This works
for repositories where the upstream source and debian/ metadata are
co-located or linked via gbp conventions.

pkg-linux-qcom does not fit this model:

• The kernel source is a shallow clone of qualcomm-linux/kernel at a
  qcom-next-* tag, optionally patched with qcom-next and kernel-topics
  PRs.
• The debian/ metadata is injected by prepare-source.sh from the
  pkg-linux-qcom packaging repository into the kernel source tree.
• The assembled tree is ephemeral — it exists only in the build workspace
  and has no corresponding git ref, no upstream tag, no pristine-tar
  branch, and no debian/gbp.conf.

Consequently:

• There is no debian-ref that can be checked out to produce a buildable
  srcpkg/.
• gbp buildpackage cannot reconstruct the .orig.tar.gz because there
  is no pristine-tar data and no upstream tag to fetch from.

Both failure modes are structural, not configuration errors. They require
new delivery and source-package-generation paths, not workarounds.

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New inputs

srcpkg-artifact — controls source tree delivery

When empty (the default), the existing behaviour is preserved: the
packaging repository is checked out at debian-ref in both the
debian-build and test jobs. No existing callers are affected.

When set, the named artifact is expected to be a tar.gz archive of the
prepared source tree. The archive is downloaded to a staging directory
and extracted with --strip-components=1 directly into srcpkg/.

Why tar.gz and not a direct directory upload?
actions/upload-artifact uses zip format internally, which strips Unix
execute permissions. Kernel build scripts (e.g. scripts/cc-version.sh)
require execute permission; without it, make defconfig fails with
Permission denied inside the Debusine build environment. tar
preserves permissions end-to-end; the zip layer wraps a single opaque
file and permissions are restored on extraction.

The debian-ref input remains required by the workflow schema but is not
consumed for checkout when srcpkg-artifact is set.

assemble-orig — controls source package generation strategy

When false (the default), generate-source-package uses gbp buildpackage
to reconstruct the orig tarball from pristine-tar or an upstream tag.

When true, passes DEBUSINE_ASSEMBLE_ORIG=true to generate-source-package
(added in qualcomm-linux/debusine-action
feat/generate-source-package-non-gbp-quilt), which:

1. Creates ../<pkg>_<upstream_ver>.orig.tar.gz directly from the source
   tree, excluding ./debian and ./.git
2. Invokes dpkg-buildpackage -S -sa -d -nc -us -uc to produce the
   .dsc, .debian.tar.xz, and .changes files

This is required for source trees that have no upstream tarball available
via gbp or pristine-tar — specifically, shallow kernel clones assembled
at build time with no version-control history that gbp can interrogate.

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Design

The two inputs are orthogonal:

• srcpkg-artifact determines where the source tree comes from
• assemble-orig determines how the source package is generated from it

A caller can use either independently, though for pkg-linux-qcom both
are required together.

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Dependency

This PR depends on:

• Add caller-controlled orig tarball assembly for externally-prepared source trees debusine-action#27:
  adds DEBUSINE_ASSEMBLE_ORIG support to generate-source-package.

Until that PR is merged into main, DEBUSINE_ACTION_REF is pinned to
feat/generate-source-package-non-gbp-quilt and will be updated to main
once merged.