ci: include all packages in coverage with -coverpkg

Without -coverpkg=./..., coverage is only counted for the package
containing the test file. This excluded 15 packages that have no
internal tests and are only exercised by the integration tests in
pkg/api/test/ and pkg/cli/test/:

  pkg/cli, pkg/pdfcpu/validate, pkg/pdfcpu/primitives,
  pkg/pdfcpu/form, pkg/pdfcpu/create, pkg/pdfcpu/sign,
  pkg/pdfcpu/font, pkg/pdfcpu/color, pkg/pdfcpu/draw,
  pkg/pdfcpu/format, pkg/pdfcpu/matrix, pkg/pdfcpu/scan,
  pkg/font, internal/corefont/metrics, cmd/pdfcpu

Co-Authored-By: Claude <noreply@anthropic.com>