The checksums are published in a transparency log, which uses a Merkle Tree[1] to make the attack you describe detectable. Source Spotter, which is unaffiliated with Google, continuously verifies that the log contains only one checksum per module version.

If Google were to present you with a different view of the Merkle Tree with different checksums in it, they'd have to forever show you, and only you, that view. If they accidentally show someone else that view, or show you the real view, the go command would detect it. This will eventually be strengthened further with witnessing[2], which will ensure that everyone's view of the log is the same. In the meantime, you / your coworker can upload your view of the log (in $GOPATH/pkg/sumdb/sum.golang.org/latest) to Source Spotter and it will tell you if it's consistent with its view:

  $ curl --data-binary "@$(go env GOPATH)/pkg/sumdb/sum.golang.org/latest" https://gossip.api.sourcespotter.com/sum.golang.org 
  consistent: this STH is consistent with other STHs that we've seen from sum.golang.org

[2] https://github.com/C2SP/C2SP/blob/main/tlog-witness.md