CI Nits and Improvements

All of these are courtesy of @endbr64.

Concurrency

All the run-on-each-PRs deserve a concurrency key to reduce resource usage on subsequent pushes:

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: ${{ github.event_name == 'pull_request' }}

Cold Compilation

govulncheck gets cold-compiled on each run.
https://github.com/letsencrypt/boulder/blob/c5f1008d88f790b7298689358dc7e24389babf21/.github/workflows/boulder-ci.yml#L118-L139

You might want to pull in actions/cache in some fashion:

# Gather the Go version for the cache key.
      - name: Record Go version
        id: go-version
        run: echo "version=$(go env GOVERSION)" >> "$GITHUB_OUTPUT"

      # Without this, every run downloads and compiles govulncheck's whole
      # dependency tree from scratch.
      - name: Cache govulncheck build
        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
        with:
          path: |
            ~/go/pkg/mod
            ~/.cache/go-build
          key: govulncheck-${{ runner.os }}-${{ steps.go-version.outputs.version }}-${{ env.GOVULNCHECK_VERSION }}
          restore-keys: |
            govulncheck-${{ runner.os }}-

      - name: Run govulncheck
        run: go run "golang.org/x/vuln/cmd/govulncheck@${GOVULNCHECK_VERSION}" ./...

Is Zizmor required to pass?

zizmor.yml isn't included in this block, despite the comment. Presumably it's because it's either allowed to soft-fail or handled by the branch protection rules:
https://github.com/letsencrypt/boulder/blob/c5f1008d88f790b7298689358dc7e24389babf21/.github/workflows/boulder-ci.yml#L169-L187

Zizmor is not being updated by Dependabot

Speaking of zizmor, using the env block to load it is opaque to Dependabot, so you're out of date (https://github.com/zizmorcore/zizmor/releases/tag/v1.30.0 is the current release). You might want to switch to using zizmorcore/zizmor-action:
https://github.com/letsencrypt/boulder/blob/c5f1008d88f790b7298689358dc7e24389babf21/.github/workflows/zizmor.yml#L16-L20

Start with .github/workflows/boulder-ci.yml, especially the run-on-each-PR jobs and the govulncheck and Zizmor sections, then compare .github/workflows/zizmor.yml. Determine which concurrency and cache changes are appropriate and whether Zizmor is required to pass. Done means the selected CI improvements are implemented and the workflow and dependency-update behavior are clear.