ci: checkout first, disable go caching

[kolyshkin]

This is an alternative to #155.

---

First, checking out the code should be done before installing Go.

Second, since we don't have top-level go.mod, actions/setup-go complains:

Restore cache failed: Dependencies file is not found in /home/runner/work/sys/sys. Supported file pattern: go.sum

One way to solve this would be to add

cache-dependency-path: "*/go.sum"

parameter to actions/setup-go. Alas it won't work because not all modules here have go.mod, and * in GHA is not what you think it is (i.e. not a part of shell glob pattern, but rather "every directory"), and as a result it complains about missing go.sum files.

Another way is to list all the paths explicitly, which is not good from the maintainability perspective (someone will definitely forgot to add a go.sum path).

Yet another way is to add a step which lists all go.sum. It works something like this:

            - name: Find go.sum files
              id: gosum
              run: |
              echo 'files<<EOF' >> "$GITHUB_OUTPUT"
              git ls-files '*/go.sum' >> "$GITHUB_OUTPUT"
              echo 'EOF' >> "$GITHUB_OUTPUT"
    
            ...
            - name: Install Go
              uses: actions/setup-go@v5
                with:
                  cache-dependency-path: ${{ steps.gosum.outputs.files }}

But given the fact that caching is not doing much here (as we don't have a lot of code) it's becoming too complicated with not much to gain.

So, let's just disable Go caching, so the warning above will go away.

[kolyshkin]

Checked that is solves the warning issue, and no attempts to save or restore cache is done.

[thaJeztah]

LGTM

left one comment about a comment, but mostly thinking out loud, and I'm fine with taking this as-is for sure

[tianon]

LGTM (a comment would be nice though, maybe even just a pointer to this PR for the trivial context reminder?)