The compiler does not need it anymore, has not needed it for many years
iirc. This just goes in and pollutes the environment overriding the
users GOPATH and causing grief.

Go even warns about it itself, without vs with this commit:

```sh
~> go env GOPATH
/home/manny/go
~> nix-shell -p go
~> go env GOPATH
warning: GOPATH set to GOROOT (/nix/store/gvw1mfpdrk7i82884yhxf9lf5j3c12zm-go-1.14.1/share/go) has no effect
/nix/store/gvw1mfpdrk7i82884yhxf9lf5j3c12zm-go-1.14.1/share/go
~> exit
~> nix-shell -I nixpkgs=cloned/NixOS/nixpkgs -p go
~> go env GOPATH
/home/manny/go
~> exit
```

(cherry picked from commit a1e13f6)

a1e13f6

(cherry picked from commit 469f14c)

(cherry picked from commit 416caeb)

https://golang.org/doc/devel/release.html#go1.14.minor
(cherry picked from commit 8d41f9f)

(cherry picked from commit d1e7b00)

https://golang.org/doc/devel/release.html#go1.13.minor
(cherry picked from commit 7da08af)

https://golang.org/doc/devel/release.html#go1.14.minor
(cherry picked from commit 9f97814)

(cherry picked from commit aae680c)

The builder does not technically need the modSha256 of the vendor dir, and even
though we pass it the entire vendor dir it makes sense not to risk having an
accidental dependency on that variable.

However, tools like [nixpkgs-update](https://github.com/ryantm/nixpkgs-update)
need to inspect the `modSha256` of a package in order to be able to update them,
and since this is a real part of the package (describes info about its
dependencies) let's add it to `passthru`.

Specifically, this allows us to run a cmd like `nix eval -f . tflint.modSha256`
to get the current value, which is how the bot finds it to replace with the new
version in the Rust ecosystem.

(cherry picked from commit 5f77ff6)

This is done in response to complaints that the module format is not
human readable. The vendor source blob is flat files and should be
extremely readable.

(cherry picked from commit 9761128)

(removed warning for 20.03)
(cherry picked from commit a0ddea1)