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

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	9m 26s
Test Suites - Darwin (no-ble-asan-clang)	1h 33m 34s
Test Suites - Darwin (no-ble-tsan-clang)	2h 9m 10s
Test Suites - Linux (no-ble-tsan-clang)	6m 46s
	3h 58m 56s

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