cmd/api: use api/next directory for beta versions

Even though we don't issue beta pre-releases of Go at this time,
it can still be useful to build them without publishing as part
of testing the release infrastructure.

For such versions, use the next directory content so that the
API check doesn't produce a false positive during the earlier
stages of the development cycle, before the next directory is
merged into a combined and eventually frozen api file.

For #29205.

Change-Id: Ib5e962670de1df22f7df64dd237b555953096808
Reviewed-on: https://go-review.googlesource.com/c/go/+/542000
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/api/main_test.go

@@ -106,7 +106,7 @@ func Check(t *testing.T) {
 	}
 
 	var nextFiles []string
-	if strings.Contains(runtime.Version(), "devel") {
+	if v := runtime.Version(); strings.Contains(v, "devel") || strings.Contains(v, "beta") {
 		next, err := filepath.Glob(filepath.Join(testenv.GOROOT(t), "api/next/*.txt"))
 		if err != nil {
 			t.Fatal(err)