fuzz: Introduce BITCOINFUZZ environment variable

The `BITCOINFUZZ` environment variable allows to override the default
path to the fuzz binary.

It complements the already existing set of variables used by tests:
 - BITCOIND
 - BITCOINCLI
 - BITCOINUTIL
 - BITCOINWALLET

test/fuzz/test_runner.py

@@ -104,7 +104,7 @@ def main():
         logging.error("Must have fuzz executable built")
         sys.exit(1)
 
-    fuzz_bin=os.path.join(config["environment"]["BUILDDIR"], 'src', 'test', 'fuzz', 'fuzz')
+    fuzz_bin=os.getenv("BITCOINFUZZ", default=os.path.join(config["environment"]["BUILDDIR"], 'src', 'test', 'fuzz', 'fuzz'))
 
     # Build list of tests
     test_list_all = parse_test_list(