VEX related improvements in the build process

[macedogm]

The VEX Hub initiative in rancher/vexhub is now a mature project in Rancher where we generate VEX reports for known false-positive CVEs in our projects (images and binaries).

In order for VEX to fully work and for security scanners (e.g., Trivy) to correctly match a VEX entry in our VEX Hub with a Go binary being scanned, the Go binary must have its full package path inside of it. Example:

> go version -m bin/rke-etcd-backup | head -n 3
bin/rke-etcd-backup: go1.23.4
	path	github.com/rancher/rke-tools
	mod	github.com/rancher/rke-tools	(devel)	

Given that the repo's full address wasn't specified in the go.mod file, the current generated path is partial.

> go version -m bin/rke-etcd-backup | head -n 3
bin/rke-etcd-backup: go1.23.4
	path	rke-tools
	mod	rke-tools	(devel)	

In the above case, the security scanner won't be able to match the binary with its respective VEX entry.

This PR isn't expected to affect the code's behavior and features, because it's only metadata information.

[macedogm]

Once the PR is merged, we should cut a new tag and update its reference in r/r and other places where it's used, please.