CI cache poisoning

[vroldanbet]

Our CI jobs running tests rely on caching to skip the whole go mod download phase, which usually eats a lot of time. Caching was introduced recently in #950, which can shove up to 2m in the best case.

Unfortunately, given we run multiple tests as individual jobs, and due to go's toolchain only downloading the modules needed as of recent versions, sometimes our CI my end up caching a version that has a small subset of modules, and as a consequence we lose the boost in CI time.

We should investigate why this happens, which job is doing this, and how can we make sure it does not write the local go empty module cache to CI cache.

[vroldanbet]

The Analyzer CI build seems to be the culprit:

Post job cleanup.
/opt/hostedtoolcache/go/1.19.[2](https://github.com/authzed/spicedb/actions/runs/3347243555/jobs/5545015920#step:7:2)/x64/bin/go env GOMODCACHE
/opt/hostedtoolcache/go/1.19.2/x64/bin/go env GOCACHE
/home/runner/go/pkg/mod
/home/runner/.cache/go-build
/usr/bin/tar --posix --use-compress-program zstd -T0 -cf cache.tzst --exclude cache.tzst -P -C /home/runner/work/spicedb/spicedb --files-from manifest.txt
Cache Size: ~12 MB (12648049 B)
Cache saved successfully
Cache saved with the key: setup-go-Linux-go-~1.19.2-ee46[3](https://github.com/authzed/spicedb/actions/runs/3347243555/jobs/5545015920#step:7:3)255313e88769c6[4](https://github.com/authzed/spicedb/actions/runs/3347243555/jobs/5545015920#step:7:4)1f1b16cb6f[5](https://github.com/authzed/spicedb/actions/runs/3347243555/jobs/5545015920#step:7:5)4cac77c811a33f2a2f952[6](https://github.com/authzed/spicedb/actions/runs/3347243555/jobs/5545015920#step:7:6)9153a1315[7](https://github.com/authzed/spicedb/actions/runs/3347243555/jobs/5545015920#step:7:7)e

[vroldanbet]

The change worked, but reopening because we still have poisoning between builds doing go build and builds doing go test, which use the same path. It's a difference of 200MB of dependencies.