go_context: set GOTOOLCHAIN to 'local' (#3660)

Enforce usage of the current toolchain and avoid downloading new
toolchains during build.

Author: sluongng

go/private/context.bzl

@@ -446,6 +446,12 @@ def go_context(ctx, attr = None):
         # Explicitly clear this environment variable to ensure that doesn't
         # happen. See #2291 for more information.
         "GOPATH": "",
+
+        # Since v1.21.0, set GOTOOLCHAIN to "local" to use the current toolchain
+        # and avoid downloading other toolchains.
+        #
+        # See https://go.dev/doc/toolchain for more info.
+        "GOTOOLCHAIN": "local",
     }
 
     # The level of support is determined by the platform constraints in