Add exec_compatible_with to @go_sdk//:builder

go_tool_binary() is completely oblivious of toolchains. The reason being
that Go toolchains actually depend on their output. Adding a toolchain
dependency would thus add a cyclic dependency. This is problematic,
because it means that the actions with mnemonic GoToolchainBinaryBuild
end up getting scheduled on arbitrary workers.

Address this by adding exec_compatible_with to the locations where
go_tool_binary() is instantiated, namely the auto-generated BUILD files
that are part of Go SDKs.

Fixes: bazelbuild#3942

go/private/BUILD.sdk.bazel

@@ -64,6 +64,7 @@ go_tool_binary(
     srcs = ["@io_bazel_rules_go//go/tools/builders:builder_srcs"],
     ldflags = "-X main.rulesGoStdlibPrefix={}".format(RULES_GO_STDLIB_PREFIX),
     sdk = ":go_sdk",
+    exec_compatible_with = {exec_compatible_with},
 )
 
 non_go_reset_target(

go/private/sdk.bzl

@@ -15,6 +15,7 @@
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "patch", "read_user_netrc", "use_netrc")
 load("//go/private:common.bzl", "executable_path")
 load("//go/private:nogo.bzl", "go_register_nogo")
+load("//go/private:platforms.bzl", "GOARCH_CONSTRAINTS", "GOOS_CONSTRAINTS")
 load("//go/private/skylib/lib:versions.bzl", "versions")
 
 MIN_SUPPORTED_VERSION = (1, 14, 0)
@@ -515,6 +516,10 @@ def _sdk_build_file(ctx, platform, version, experiments):
             "{exe}": ".exe" if goos == "windows" else "",
             "{version}": version,
             "{experiments}": repr(experiments),
+            "{exec_compatible_with}": repr([
+                GOARCH_CONSTRAINTS[goarch],
+                GOOS_CONSTRAINTS[goos],
+            ]),
         },
     )