cmd/release: use macOS 10.15 (Catalina) for releases

This builder has Xcode installed instead of just the command-line
tools. Because the SDKs are absent when just the command-line tools
are installed, no base SDK version is being set in the go binary
(it uses external linking) and codesign was printing a warning about
library validation. Updating to use this builder gets rid of that
warning.

Updates golang/go#35459

Change-Id: Ib548c998042f499c2d57324e9a8746594ea7060d
Reviewed-on: https://go-review.googlesource.com/c/build/+/208268
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
diff --git a/cmd/release/release.go b/cmd/release/release.go
index db15f7e..cb7d6e0 100644
--- a/cmd/release/release.go
+++ b/cmd/release/release.go
@@ -194,7 +194,7 @@
 		OS:      "darwin",
 		Arch:    "amd64",
 		Race:    true,
-		Builder: "darwin-amd64-10_11",
+		Builder: "darwin-amd64-10_15",
 	},
 	{
 		OS:       "linux",