podman build --build-arg should fall back to environment

Fixes: https://github.com/containers/podman/issues/9571

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

cmd/podman/images/build.go

@@ -318,7 +318,12 @@ func buildFlagsWrapperToOptions(c *cobra.Command, contextDir string, flags *buil
 			if len(av) > 1 {
 				args[av[0]] = av[1]
 			} else {
-				delete(args, av[0])
+				// check if the env is set in the local environment and use that value if it is
+				if val, present := os.LookupEnv(av[0]); present {
+					args[av[0]] = val
+				} else {
+					delete(args, av[0])
+				}
 			}
 		}
 	}