Allow to build runtime from custom sdk

[sergio-costas]

This patch adapts the script from gnome-46-2404 that allows to build the Runtime from a custom, prebuilt SDK. It is needed to be able to test any PR in the SDK using CI.

It also fixes the CRAFT_ARCH_TRIPLET warnings.

[Copilot]

Pull request overview

This PR adds support for building the gnome-42-2204 runtime snap against a custom, prebuilt SDK snap (e.g., for CI validation of SDK PRs), instead of always pulling the SDK from the Snap Store.

Changes:

• Extend snap/snapcraft.yaml to optionally consume a locally provided SDK snap during the gnome-sdk part build.
• Add local-build.py to generate a temporary modified snapcraft configuration and run a local build (or prepare-only mode for CI).
• Expand README.md with usage documentation for building with a local SDK snap.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
snap/snapcraft.yaml	Adds conditional logic to use a local SDK snap and adjusts architecture-triplet path usage.
README.md	Documents the new local SDK build workflow and the local-build.py script.
local-build.py	New helper script to generate a modified snapcraft config and drive local/CI builds using a local SDK snap.

Comments suppressed due to low confidence (1)

snap/snapcraft.yaml:56

• When LOCAL_SDK_SNAP is set, this branch doesn’t set sdk_version in the script itself, but it’s used later in version="${project_version}-sdk${sdk_version}". With set -u, the build depends on sdk_version being provided via the part’s environment (e.g., by local-build.py). Consider deriving it from the local snap here (or explicitly validating it) to make the snapcraft build logic self-contained and less fragile.

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