ci: keep vcpkg.json in Dependabot's format and out of the linters (#2774)
Dependabot rewrites `vcpkg.json` with expanded arrays and no final
newline, so every baseline bump fails Prettier and EditorConfig (see
https://github.com/php/frankenphp/pull/2773).
- Write `vcpkg.json` exactly as Dependabot does, so its PRs only change
the `builtin-baseline` line.
- Exclude `vcpkg.json` from super-linter. `vcpkg install` still rejects
an invalid manifest.