Tweak how we pass GOEXPERIMENT to actions (#4022)

**What type of PR is this?**
Small cleanup. We can just encode the string in the repo rule instead of
creating it for every go action.

**What does this PR do? Why is it needed?**

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

Author: dzbarsky

go/private/BUILD.sdk.bazel

@@ -62,9 +62,9 @@ go_sdk(
 go_tool_binary(
     name = "builder",
     srcs = ["@io_bazel_rules_go//go/tools/builders:builder_srcs"],
+    exec_compatible_with = {exec_compatible_with},
     ldflags = "-X main.rulesGoStdlibPrefix={}".format(RULES_GO_STDLIB_PREFIX),
     sdk = ":go_sdk",
-    exec_compatible_with = {exec_compatible_with},
 )
 
 non_go_reset_target(

go/private/context.bzl

@@ -350,7 +350,7 @@ def _collect_cc_infos(deps, cdeps):
 def _check_binary_dep(go, dep, edge):
     """Checks that this rule doesn't depend on a go_binary or go_test.
 
-    go_binary and go_test may return provides with useful information for other
+    go_binary and go_test may return providers with useful information for other
     rules (like go_path), but go_binary and go_test may not depend on other
     go_binary and go_binary targets. Their dependencies may be built in
     different modes, resulting in conflicts and opaque errors.
@@ -483,7 +483,7 @@ def go_context(ctx, attr = None):
     env = {
         "GOARCH": mode.goarch,
         "GOOS": mode.goos,
-        "GOEXPERIMENT": ",".join(toolchain.sdk.experiments),
+        "GOEXPERIMENT": toolchain.sdk.experiments,
         "GOROOT": goroot,
         "GOROOT_FINAL": "GOROOT",
         "CGO_ENABLED": "0" if mode.pure else "1",

go/private/providers.bzl

@@ -41,7 +41,7 @@ GoSDK = provider(
     fields = {
         "goos": "The host OS the SDK was built for.",
         "goarch": "The host architecture the SDK was built for.",
-        "experiments": "Go experiments to enable via GOEXPERIMENT.",
+        "experiments": "Comma-separated Go experiments to enable via GOEXPERIMENT.",
         "root_file": "A file in the SDK root directory",
         "libs": ("Depset of pre-compiled .a files for the standard library " +
                  "built for the execution platform."),

go/private/rules/sdk.bzl

@@ -47,9 +47,9 @@ go_sdk = rule(
             mandatory = True,
             doc = "The host architecture the SDK was built for",
         ),
-        "experiments": attr.string_list(
+        "experiments": attr.string(
             mandatory = False,
-            doc = "Go experiments to enable via GOEXPERIMENT",
+            doc = "Comma-separated Go experiments to enable via GOEXPERIMENT",
         ),
         "root_file": attr.label(
             mandatory = True,

go/private/sdk.bzl

@@ -515,7 +515,7 @@ def _sdk_build_file(ctx, platform, version, experiments):
             "{goarch}": goarch,
             "{exe}": ".exe" if goos == "windows" else "",
             "{version}": version,
-            "{experiments}": repr(experiments),
+            "{experiments}": repr(",".join(experiments)),
             "{exec_compatible_with}": repr([
                 GOARCH_CONSTRAINTS[goarch],
                 GOOS_CONSTRAINTS[goos],