Configure environment to avoid toolchain installs

Force `go` to always use the local toolchain (i.e. the one the one that
shipped with the go command being run) via setting the `GOTOOLCHAIN`
environment variable to `local`[1]:

> When GOTOOLCHAIN is set to local, the go command always runs the
bundled Go toolchain.

This is how things are setup in the official Docker images (e.g.[2], see
also the discussion around that change[3]). The motivation behind this
is to:

* Reduce duplicate work, the action will install a version of Go, a
  toolchain will be detected, the toolchain will be detected and then
  another version of Go installed
* Avoid Unexpected behaviour: if you specify this action runs with some Go
  version (e.g. `1.21.0`) but your go.mod contains a `toolchain` or `go`
  directive for a newer version (e.g. `1.22.0`) then, without any other
  configuration/environment setup, any go commands will be run using go
  `1.22.0`
* TODO: link image

This will be a **breaking change** for some workflows. Given a `go.mod`
like:

    module proj

    go 1.22.0

Then running any `go` command, e.g. `go mod tidy`, in an environment
where only go versions before `1.22.0` were installed would previously
trigger a toolchain download of Go `1.22.0` and that version being used
to execute the command. With this change the above would error out with
something like:

> go: go.mod requires go >= 1.22.0 (running go 1.21.7;
GOTOOLCHAIN=local)

[1] https://go.dev/doc/toolchain#select
[2] dae3405a32/Dockerfile-linux.template (L163)
[3] https://github.com/docker-library/golang/issues/472

dist/setup/index.js

@@ -88649,6 +88649,7 @@ const os_1 = __importDefault(__nccwpck_require__(2037));
 function run() {
     return __awaiter(this, void 0, void 0, function* () {
         try {
+            setToolchain();
             //
             // versionSpec is optional.  If supplied, install / use from the tool cache
             // If not supplied then problem matchers will still be setup.  Useful for self-hosted.
@@ -88765,6 +88766,21 @@ function resolveVersionInput() {
     }
     return version;
 }
+function setToolchain() {
+    // docs: https://go.dev/doc/toolchain
+    // "local indicates the bundled Go toolchain (the one that shipped with the go command being run)"
+    // this is so any 'go' command is run with the selected Go version
+    // and doesn't trigger a toolchain download and run commands with that
+    // see e.g. issue #424
+    // and a similar discussion: https://github.com/docker-library/golang/issues/472
+    const toolchain = 'local';
+    const toolchainVar = 'GOTOOLCHAIN';
+    // set the value in process env so any `go` commands run as child-process
+    // don't cause toolchain downloads
+    process.env[toolchainVar] = toolchain;
+    // and in the runner env so e.g. a user running `go mod tidy` won't cause it
+    core.exportVariable(toolchainVar, toolchain);
+}
 
 
 /***/ }),