[Linux-SGX] Makefile.Test: define SGX_SIGNER_KEY only when not defined

Pal/src/host/Linux-SGX/Makefile.Test

@@ -2,7 +2,7 @@ SGX_DIR = $(abspath $(dir $(lastword $(MAKEFILE_LIST))))
 RUNTIME_DIR = $(SGX_DIR)/../../../../Runtime
 
 LIBPAL = $(RUNTIME_DIR)/libpal-Linux-SGX.so
-SGX_SIGNER_KEY = $(SGX_DIR)/signer/enclave-key.pem
+SGX_SIGNER_KEY ?= $(SGX_DIR)/signer/enclave-key.pem
 SGX_SIGN = $(SGX_DIR)/signer/pal-sgx-sign -libpal $(LIBPAL) -key $(SGX_SIGNER_KEY)
 SGX_GET_TOKEN = $(SGX_DIR)/signer/pal-sgx-get-token