Depending on jdk:toolchain causes java_grpc_library to always use the
_default_ toolchain, even if the user tried to override it. Changing to
:current_java_toolchain allows the rule to use the user-selected
toolchain when overridden.

Tested by adding to BUILD:
load("@bazel_tools//tools/jdk:default_java_toolchain.bzl", "default_java_toolchain")
default_java_toolchain(
    name = "mychain",
    misc = ["-Amy=flag"],
    visibility = ["//visibility:public"],
)

And then verifying -Amy=flag is in the output of:
bazel aquery --java_toolchain=:mychain services:_reflection_java_grpc

Fixes grpc#5841