fix(garm): build openstack provider as static binary

[cbartz]

What this PR does

Builds the garm-provider-openstack binary in the GARM rock with CGO_ENABLED=0 so it is a fully static, pure-Go binary, and drops the ineffective -extldflags -static ldflag.

Why we need it

On the deployed garm charm, runner consolidation failed with:

provider binary /usr/local/bin/garm-provider-openstack ... fork/exec: no such file or directory

The binary was present but dynamically linked: with cgo enabled (default) and the net package imported, Go links glibc's NSS resolvers dynamically via its internal linker, which never invokes the external linker — so the existing -extldflags -static was silently ignored. The rock uses a bare base with no dynamic loader/libc, so GARM could not exec it and no runners were spawned — while the charm still reported active (the charm doesn't scrape GARM's runtime logs). The garm binary avoids this via its osusergo,netgo tags; the provider had neither those nor CGO_ENABLED=0.

Test plan

• Reproduced the original build locally (go build -ldflags "-extldflags -static", cgo default) → dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2.
• Verified the fix (CGO_ENABLED=0 go build) → statically linked, no ELF interpreter.
• Rebuilt the rock end-to-end (build-garm-rock.sh), extracted the provider from the OCI layer → confirmed statically linked, no interp.

Review focus

• Chose CGO_ENABLED=0 over adding osusergo,netgo tags: the OpenStack provider is pure Go with no cgo dependency (unlike garm's sqlite), so disabling cgo outright is the simplest guarantee and removes any reliance on the external linker honouring -static.

Potential breaking changes / new dependencies

None. Build-only change; no new modules, APIs, or workflow changes.

Checklist

• Changes comply with the project's coding standards and guidelines (see CONTRIBUTING.md and STYLE.md)
• CONTRIBUTING.md has been updated upon changes to the contribution/development process (e.g. changes to the way tests are run) — N/A
• Technical author has been assigned to review the PR in case of documentation changes (usually *.md files) — N/A, only a changelog entry
• I updated docs/changelog.md with user-relevant changes
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration) — N/A; static-linking is a rock build property with no charm-level test surface
• If integration test modules are used: I updated the workflow configuration — N/A
• If this PR involves a Grafana dashboard: I added a screenshot of the dashboard — N/A
• If this PR involves Terraform: terraform fmt passes and tflint reports no errors — N/A
• If this PR involves Rockcraft: I updated the version — the garm rock version is static at 0.1 and prior rock fixes (e.g. fix(garm): add CA trust store to bare rock so GARM can verify TLS #257) did not bump it; left unchanged for consistency
• If this PR adds/removes a charm, or changes a charm's base class, conventions, tooling, or repo structure: I updated the relevant AGENTS.md — N/A
• If this PR changes .copilot-collections.yaml or .github/instructions/: I re-checked whether the AGENTS.md "12-factor divergences" guidance still matches the upstream copilot-collections guidance — N/A

[Copilot]

Pull request overview

This PR adjusts the GARM rock build so the garm-provider-openstack binary is built as a fully static (pure-Go) executable by forcing CGO_ENABLED=0, addressing runtime failures on the bare base where dynamically linked binaries cannot be exec’d due to the missing ELF interpreter.

Changes:

• Set CGO_ENABLED=0 for the garm-provider-openstack part and remove the ineffective -extldflags -static usage for that binary.
• Add a changelog entry documenting the runner provisioning fix and the root cause.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
garm-rockcraft.yaml	Forces a pure-Go, static build of garm-provider-openstack suitable for base: bare and removes an ldflag that was not achieving static linking.
docs/changelog.md	Documents the user-visible impact and the build change that fixes provider execution.