ci: cache is not being used properly

The following discussion from !1175 (merged) should be addressed:

@jaime started a discussion: (+2 comments)
We set $GOPATH/pkg/mod/ as a dir for the cache, however this is not set in the image we use for some reason (or perhaps it is but the structure is different now?). Most jobs failed to save the cache because /pkg/mod doesn't exist
```
Creating cache retryable-tests-1-non_protected...
74WARNING: /pkg/mod/: no matching files. Ensure that the artifact path is relative to the working directory 
75WARNING: bin/: no matching files. Ensure that the artifact path is relative to the working directory 
```
https://gitlab.com/gitlab-org/container-registry/-/jobs/3476582762