fix: prevent noclobber issues on /dev/null

This seems like the reason for the jenv-shim being left behind i.e.
rehash is run, the shim is created but we can't redirect to stderr
because of noclobber. This causes rehash to exit because of set -e but
does so before the trap is established to clean up the shim. Further
rehash executions fail because of the existence of the shim.

See https://github.com/rbenv/rbenv/issues/759#issuecomment-289326891

libexec/jenv-rehash

@@ -30,7 +30,7 @@ fi
 # to stderr and exit with a non-zero status.
 set -o noclobber
 { echo > "$PROTOTYPE_SHIM_PATH"
-} 2>/dev/null ||
+} 2>| /dev/null ||
 { echo "jenv: cannot rehash: $PROTOTYPE_SHIM_PATH exists, try 'jenv rehash --force' to override"
   exit 1
 } >&2