Fix use of `//go` with Bzlmod by removing dependency on `@org_golang_x_sys` (#3512)

* Fix use of `//go` by exposing `@org_golang_x_sys` when using Bzlmod

Running `bazel run //go` (or `bazel run @rules_go//go` from another module) fails when using Bzlmod because the `@org_golang_x_sys` repository is not visible. Without Bzlmod, the repository is loaded through a WORKSPACE dependency and the error does not happen.

Two-step minimal reproduction from root of this repo:

1. `touch WORKSPACE.bzlmod`
2. `bazel run --enable_bzlmod //go`

Here's the error that produces:

```shell
$ bazel run --enable_bzlmod //go
DEBUG: gazelle@0.27.0/MODULE.bazel:7:6: WARNING: The bazel_gazelle Bazel module is still highly experimental and subject to change at any time. Only use it to try out bzlmod for now.
WARNING: For repository 'com_google_protobuf', the root module requires module version protobuf@3.19.2, but got protobuf@3.19.6 in the resolved dependency graph.
ERROR: /Users/brice/Documents/Code/rules_go/go/tools/go_bin_runner/BUILD.bazel:11:11: no such package '@[unknown repo 'org_golang_x_sys' requested from @]//unix': The repository '@[unknown repo 'org_golang_x_sys' requested from @]' could not be resolved: No repository visible as '@org_golang_x_sys' from main repository and referenced by '//go/tools/go_bin_runner:go_bin_runner_lib'
ERROR: Analysis of target '//go:go' failed; build aborted: 
INFO: Elapsed time: 3.043s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (45 packages loaded, 865 targets configured)
    currently loading: go/platform
ERROR: Build failed. Not running target
```

Here's the point in the BUILD file where this repository is being referenced:

https://github.com/bazelbuild/rules_go/blob/e558f56df9995f571e49de336c3831d407d9ff70/go/tools/go_bin_runner/BUILD.bazel#L25

* Remove `process_unix.go` and use `process.go` for all platforms. Eliminates dependency on `@org_golang_x_sys`.

* Add Bzlmod test for //go

---------

Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>

1 files changed, 0 insertions, 2 deletions

diff --git a/go/tools/go_bin_runner/process.go b/go/tools/go_bin_runner/process.go
index 8b344c96..af236549 100644
--- a/go/tools/go_bin_runner/process.go
+++ b/go/tools/go_bin_runner/process.go
@@ -1,5 +1,3 @@
-//go:build !unix
-
 package main
 
 import (