[fuzzing] Only use docker image on x86_64 machines

The echidna docker image does not support ARM
Macs, for example.

fuzzing/fuzz.sh

@@ -2,6 +2,7 @@
 set -e
 
 root=$(cd $(dirname "$0")/.. && pwd)
+arch=$(arch)
 
 if command -v echidna; then
   fuzz () {
@@ -11,7 +12,7 @@ if command -v echidna; then
       --crytic-args --ignore-compile \
       --contract $1
   }
-else
+elif [ "${arch}" = "x86_64" ]; then
   fuzz () {
     docker run \
       --rm \
@@ -23,6 +24,10 @@ else
           --crytic-args --ignore-compile \
           --contract $1"
   }
+else
+  echo "Error: echidna not found, and the docker image does not support ${arch}"
+  echo "Please install echidna: https://github.com/crytic/echidna#installation"
+  exit 1
 fi
 
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