f0aaf7095d
Our docs and default value for the GitHub action cache URL were referring to the [wrong](https://github.com/tonistiigi/go-actions-cache/blob/master/api.md) environment variable. I was under the impression that the underlying library ends up checking for the correct variable later so this would be only cosmetic, but a user [confirmed](https://github.com/pulumi/pulumi-docker-build/issues/77#issuecomment-2135621983) this does appear to break things. I brought over an old test I had written while this was still in the pulumi-docker repo, but unfortunately it doesn't currently test GHA due to ci-mgmt limitations https://github.com/pulumi/pulumi-docker-build/issues/82. Fixes https://github.com/pulumi/pulumi-docker-build/issues/77.