### Brief description of Pull Request

I'm bumping the Linux and Windows build image's Go runtime to 1.26.5 to
fix [GO-2026-5856](https://pkg.go.dev/vuln/GO-2026-5856) (crypto/tls)
and [GO-2026-4970](https://pkg.go.dev/vuln/GO-2026-4970) (os), flagged
by govulncheck on the [PR #6658 lint
job](https://github.com/grafana/alloy/actions/runs/28966883772/job/85952218978?pr=6658).
Step 2 as a follow up once the build image lands.

### PR Checklist

- [x] This pull request was substantially generated with AI assistance
(see the [GenAI
policy](https://github.com/grafana/alloy/blob/main/docs/developer/genai.md))