Pigweed python env setup now uses the "arm-none-eabi-gcc" toolchain
to derive some flags for the newly added clang for ARM toolchain.
Bug: https://crbug.com/oss-fuzz/30513
Do not run example fuzzers in pw_fuzzer as those are for demonstration
purposes.
Test: run `python infra/helper.py build_fuzzers pigweed` and observe
pw_fuzzer/toy_fuzzer in pw_fuzzer is NOT copied to out directory.
Ali Zhang