Disable various Go features via environment variables #5477
Description
Go 1.21 introduced a feature which automatically downloads toolchain binaries from the Internet if the Go version mentioned in go.mod is different from the current toolchain. This breaks our assumption that CI will run the version of the toolchain we've specified (and, in Docker containers, cryptographically verified), so we need to override it in our Makefile, CI jobs, and Docker containers using the appropriate environment variable.
While we're at it, we should also look at various other options that we can use to configure Go via the environment and determine if we need to set any of them. We should consider specifically any potential privacy implications of users, as Go uses Google-provided services and we may not want users running our Makefile to automatically send data to Google without intentionally opting in to do so.