Add defer statement

Author: egregius313

go/ql/test/library-tests/semmle/go/dataflow/flowsources/local/environment/test.expected

@@ -4,3 +4,4 @@
 | test.go:34:29:34:32 | &... |
 | test.go:41:2:41:40 | ... := ...[0] |
 | test.go:48:2:48:52 | ... := ...[0] |
+| test.go:61:2:61:33 | ... := ...[0] |

go/ql/test/library-tests/semmle/go/dataflow/flowsources/local/environment/test.go

@@ -56,7 +56,8 @@ func godotenvEnvironmentVariables() {
 }
 
 func envparseEnvironmentVariables() {
-	f := os.Open("file.txt")
+	f := os.OpenRead("file.txt")
+	defer f.Close()
 	envVars, ok := envparse.Parse(f)
 
 	if !ok {