Make GitHub suggestions for fixing sha256 fields

[Gabor Greif]

Now that GitHub has suggestion review comments, a very helpful and actionable feedback can be given to PR submitters when any field (e.g. sha256) is provably wrong, with a known correct value.

Something like a bot comment with suggestion:

-   sha256 = "1gl4fzakjbhd94v1saxmr9sfzgk22m1b95jq51rxm93b2g4cixl4";
+   sha256 = "02qa6wgjpxgakg7hv4zfdlrx9k7zxa5i02wnr6y9fsv8j16sbkh1";

This comes from the experience with NixOS/nixpkgs#77986, in which I first (knowingly) entered a wrong hash, in the hope to get a build error because of the mismatch. Which I would then track down to find the correct value in the build log.

The build finished as green soon, because of "Fixed-output derivation". Then I had various false starts supplying the expected hash.