test: make -all_codegen default to true on linux-amd64 builder

Fixes #34297

Change-Id: I4584a97d4562d7af0412d683ba1c206e3c1d9edb
Reviewed-on: https://go-review.googlesource.com/c/go/+/197539
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/run.go

@@ -34,7 +34,7 @@ var (
 	keep           = flag.Bool("k", false, "keep. keep temporary directory.")
 	numParallel    = flag.Int("n", runtime.NumCPU(), "number of parallel tests to run")
 	summary        = flag.Bool("summary", false, "show summary of results")
-	allCodegen     = flag.Bool("all_codegen", false, "run all goos/goarch for codegen")
+	allCodegen     = flag.Bool("all_codegen", defaultAllCodeGen(), "run all goos/goarch for codegen")
 	showSkips      = flag.Bool("show_skips", false, "show skipped tests")
 	runSkips       = flag.Bool("run_skips", false, "run skipped tests (ignore skip and build tags)")
 	linkshared     = flag.Bool("linkshared", false, "")
@@ -45,6 +45,14 @@ var (
 	shards = flag.Int("shards", 0, "number of shards. If 0, all tests are run. This is used by the continuous build.")
 )
 
+// defaultAllCodeGen returns the default value of the -all_codegen
+// flag. By default, we prefer to be fast (returning false), except on
+// the linux-amd64 builder that's already very fast, so we get more
+// test coverage on trybots. See https://golang.org/issue/34297.
+func defaultAllCodeGen() bool {
+	return os.Getenv("GO_BUILDER_NAME") == "linux-amd64"
+}
+
 var (
 	goos, goarch string