Less fragile way of deciding whether to run old awk test

docker-compose.yml

@@ -88,6 +88,7 @@ services:
       GETSSL_HOST: ubuntu18-no-gawk.getssl.test
       GETSSL_IP: 10.30.50.6
       NGINX_CONFIG: /etc/nginx/sites-enabled/default
+      TEST_AWK: "yes"
     networks:
       acmenet:
         ipv4_address: 10.30.50.6

test/5-old-awk-error.bats

@@ -14,8 +14,7 @@ setup() {
 @test "Check getssl fails if an old version of awk is installed" {
     CONFIG_FILE="getssl-http01.cfg"
     # Make sure this test only runs on an image running an old version of awk
-    awk_version=$(awk -V 2>/dev/null) || true
-    if [[ "$awk_version" == "" ]]; then
+    if [[ "$TEST_AWK" != "" ]]; then
         setup_environment
         init_getssl
         create_certificate