x/build/cmd/releasebot: improve -dry-run -mode=prepare to support advance testing for more release types

[dmitshur]

Right now, it's possible to use releasebot -dry-run -mode=prepare (notably, with -dry-run flag) to run a release locally that doesn't publish any public artifacts, but only for the beta release type (e.g., go1.15beta2).

For most other release types, it fails "too early" to produce useful information. For example, it doesn't take into account that the Gerrit CL updating the VERSION file isn't actually mailed, so trying to fetch its commit hash from Gerrit will give 404, and release won't be able to proceed.

This is the tracking issue about improving that, so fewer ad-hoc changes need to be made to releasebot each time this ability is needed. It's broken out of #29252 (comment).

/cc @toothrot @cagedmantis @andybons

[dmitshur]

releasebot has been superseded by relui and this issue doesn't apply directly. relui has a "dry-run (test and build only): ..." workflows for testing, and #47407 is for more. Closing.