patch: add bld support in snap builds

[skourta]

Adds optional sd build support to the build_snap.yaml reusable workflow. When enabled, the workflow installs craft-sd and bld and exposes the CI / CRAFT_SD_ENV environment variables to the snapcraft pack step so artifacts can be pulled from the correct environment.

Context

• This support is opt-in via a new sd-build input (default false), so existing consumers of the workflow are unaffected.

Changes

• New inputs in .github/workflows/build_snap.yaml:
  • sd-build (boolean, default false) — installs the required build tools (craft-sd from latest/beta, bld from edge) when true.
  • craft_sd_env (string, default production) — the environment to pull artifacts from. Only used when sd-build is true.
• New step Set up sd build environment (gated on inputs.sd-build) that installs the craft-sd and bld snaps.
• Pack step env now sets:
  • CI to 1 when sd-build is enabled (empty otherwise) (hack to make arm builds work as well).
  • CRAFT_SD_ENV to inputs.craft_sd_env when sd-build is enabled (empty otherwise).
• Minor whitespace/quote style cleanups in the same file (trailing-space removal, single→double quotes for the matrix job name, comment alignment).

Backward compatibility

Fully backward compatible: the new inputs default to false / production, so workflows that do not set sd-build behave exactly as before — no SD tools are installed and no extra env vars are passed to snapcraft.

Testing

Tested here.

[Gu1nness]

All good.
Suggestion for the craft_sd_env to make it better but approving already on the current implementation that does the job.

[Gu1nness]

I would use the type choice here to limit the available environments.
There are only 3 possible values so let's enforce it.

Suggested change

	craft_sd_env:
	description: |
	the env to use to pull artifacts from
	This is only used if `sd-build` is true.
	default: production
	type: string
	craft_sd_env:
	description: |
	the env to use to pull artifacts from
	This is only used if `sd-build` is true.
	default: production
	type: choice
	options:
	- prod
	- production
	- staging

[Gu1nness]

Nit / Non blocking: Although this works, I think this is not easy to understand that it does.
Any idea to make it more understandable ?

[carlcsaposs-canonical]

https://chat.canonical.com/canonical/pl/ozg9sf9up3g88ysrw39jqgppay (moved to Mattermost since discussion contains some non-public information)

[skourta]

We chose a different approach. Check valkey-artifacts for details.