[build]Fix FRR build cache issue

[dgsudharsan]

Why I did it

When FRR is built with Cache enabled, the build failed with the following error logs

[2023-09-20T15:17:00.273Z] fatal: Unable to hash src/sonic-frr/frr/tests/topotests/grpc_basic/lib
[2023-09-20T15:17:00.273Z] fatal: Unable to hash src/sonic-frr/frr/tests/topotests/ospfapi/lib
[2023-09-20T15:17:00.273Z] make: *** [Makefile.cache:528: target/debs/bullseye/frr_8.5.1-sonic-0_amd64.deb.smdep] Error 123
[2023-09-20T15:17:00.273Z] make: *** Waiting for unfinished jobs....

Work item tracking

• Microsoft ADO (number only):

How I did it

Currently symlinks are excluded in hardcoded fashion. With FRR upgrades new symlinks might get introduced. To overcome it modified the way in which symlinks are excluded by finding symlinks using find command

How to verify it

Build FRR with cache enabled

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)