[bug] keys goos and goarch in .slsa-goreleaser.yml do not evaluate Go template

[maruel]

Describe the bug
While the example at https://github.com/slsa-framework/slsa-github-generator/tree/main/internal/builders/go#configuration-file shows that Go templates are evaluated in the key ldflags, this is not the case for keys goos and goarch.

To Reproduce
Steps to reproduce the behavior:

Use this form in .slsa-goreleaser.yml:

goos: "{{.Env.GOOS}}"

Use this form in the github actions workflow:

jobs:
  build:
    (...)
    strategy:
      matrix:
        os:
          - darwin
          - linux
          - window
        arch:
          - amd64
          - arm64
    uses: slsa-framework/slsa-github-generator/.github/workflows/builder_go_slsa3.yml@v1.9.0
    with:
      go-version: 1.21
      evaluated-envs: "GOOS:${{matrix.os}}, GOARCH:${{matrix.arch}}"  

Expected behavior
A single .slsa-goreleaser.yml can be used even for cross-compiling.

[ianlewis]

Related to #63