internal/releasetargets: set LongTestBuilder for darwin-amd64

A longtest builder for the darwin/amd64 port was added in the Go 1.21
development cycle. Start using it as for non-advisory release testing.

For golang/go#40561.
For golang/go#35678.
For golang/go#29252.

Change-Id: Ic65e84e5e10bcb786cb28c36c1d1b137e2f6202e
Reviewed-on: https://go-review.googlesource.com/c/build/+/503758
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

Author: dmitshur

internal/releasetargets/releases.txt

@@ -104,6 +104,7 @@ Targets for release 1.21
 aix-ppc64       aix        ppc64      (cross-compiled, no tests)
 
 darwin-amd64    darwin     amd64      darwin-amd64-13
+	Long tests on darwin-amd64-longtest
 	Running on reverse builder: AWS macOS Ventura (13) VM under QEMU
 
 darwin-arm64    darwin     arm64      darwin-arm64-12

internal/releasetargets/releasetargets.go

@@ -157,6 +157,7 @@ var allReleases = map[int]ReleaseTargets{
 	21: {
 		"darwin-amd64": &Target{
 			Builder:         "darwin-amd64-13",
+			LongTestBuilder: "darwin-amd64-longtest",
 			MinMacOSVersion: "10.15", // go.dev/issue/57125
 		},
 		"linux-s390x":   nil,