Bump third_party/pigweed/repo from `d5fcc90` to `6ad0bec` #339

Job	Run time
Unit / Integration Tests (main)	21m 12s
Unit / Integration Tests (clang)	22m 29s
Unit / Integration Tests (mbedtls)	21m 6s
Unit / Integration Tests (rotating_device_id)	21m 46s
Unit / Integration Tests (icd)	54m 18s
	2h 20m 51s

Provide feedback