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

__tests__/setup-go.test.ts

@@ -265,7 +265,7 @@ describe('setup-go', () => {
     expect(logSpy).toHaveBeenCalledWith(`Setup go version spec 1.13.0`);
   });
 
-  it('does not export any variables for Go versions >=1.9', async () => {
+  it('does not export GOROOT for Go versions >=1.9', async () => {
     inputs['go-version'] = '1.13.0';
     inSpy.mockImplementation(name => inputs[name]);
 
@@ -278,7 +278,7 @@ describe('setup-go', () => {
     });
 
     await main.run();
-    expect(vars).toStrictEqual({});
+    expect(vars).not.toHaveProperty('GOROOT');
   });
 
   it('exports GOROOT for Go versions <1.9', async () => {
@@ -294,9 +294,7 @@ describe('setup-go', () => {
     });
 
     await main.run();
-    expect(vars).toStrictEqual({
-      GOROOT: toolPath
-    });
+    expect(vars).toHaveProperty('GOROOT', toolPath);
   });
 
   it('finds a version of go already in the cache', async () => {
@@ -966,4 +964,18 @@ use .
       }
     );
   });
+
+  it('exports GOTOOLCHAIN and sets it in current process env', async () => {
+    inputs['go-version'] = '1.21.0';
+    inSpy.mockImplementation(name => inputs[name]);
+
+    const vars: {[key: string]: string} = {};
+    exportVarSpy.mockImplementation((name: string, val: string) => {
+      vars[name] = val;
+    });
+
+    await main.run();
+    expect(vars).toStrictEqual({GOTOOLCHAIN: 'local'});
+    expect(process.env).toHaveProperty('GOTOOLCHAIN', 'local');
+  });
 });