Bump third_party/pigweed/repo from `1f12d06` to `7940ef2` #2416

Job	Run time
Unit / Integration Tests (main)	1s
Unit / Integration Tests (clang)	1m 59s
Unit / Integration Tests (mbedtls)	1m 20s
Unit / Integration Tests (rotating_device_id)	12s
Unit / Integration Tests (icd)	1m 57s
	5m 29s

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