make presence of register to trigger multiline script

this is similar to how it handled a single-line script with export

pkg/config/command.go

@@ -84,8 +84,9 @@ func (cmd *Cmd) GetScript() (command string, rdr io.Reader) {
 	}
 
 	elems := strings.Split(cmd.Script, "\n")
-	// export should be treated as multiline for env vars to be set
-	if len(elems) > 1 || strings.Contains(cmd.Script, "export") {
+	// If there are multiple lines, we need to use a script file.
+	// Export presence should be treated as multiline for env vars to be set. The same thing for register variables.
+	if len(elems) > 1 || strings.Contains(cmd.Script, "export") || len(cmd.Register) > 0 {
 		log.Printf("[DEBUG] command %q is multiline, using script file", cmd.Name)
 		return "", cmd.scriptFile(cmd.Script, cmd.Register)
 	}

pkg/config/command_test.go

@@ -216,6 +216,22 @@ echo 'Goodbye, World!'`,
 				"echo setvar GREETING=${GREETING}",
 			},
 		},
+		{
+			name: "single line with register",
+			cmd: &Cmd{
+				Script: `echo 'Hello, World!'
+echo 'Goodbye, World!'`,
+				Register: []string{"foo"},
+			},
+			expectedScript: "",
+			expectedContents: []string{
+				"#!/bin/sh",
+				"set -e",
+				"echo 'Hello, World!'",
+				"echo 'Goodbye, World!'",
+				"echo setvar foo=${foo}",
+			},
+		},
 	}
 
 	for _, tc := range testCases {