Skip exec.LookPath if a specific gzip implementation is disabled

Both pigz and igzip can be disabled via the environment variables.
If disabled, calling exec.LookPath and logging "not found" message is,
even in the debug level, doesn't make much sense.

Signed-off-by: Kazuyoshi Kato <kaz@fly.io>

archive/compression/compression.go

@@ -303,27 +303,24 @@ func cmdStream(cmd *exec.Cmd, in io.Reader) (io.ReadCloser, error) {
 }
 
 func detectCommand(path, disableEnvName string) string {
+	// Check if this command is disabled via the env variable
+	value := os.Getenv(disableEnvName)
+	if value != "" {
+		disable, err := strconv.ParseBool(value)
+		if err != nil {
+			log.L.WithError(err).Warnf("could not parse %s: %s", disableEnvName, value)
+		}
+
+		if disable {
+			return ""
+		}
+	}
+
 	path, err := exec.LookPath(path)
 	if err != nil {
 		log.L.WithError(err).Debugf("%s not found", path)
 		return ""
 	}
 
-	// Check if this command is disabled via the env variable
-	value := os.Getenv(disableEnvName)
-	if value == "" {
-		return path
-	}
-
-	disable, err := strconv.ParseBool(value)
-	if err != nil {
-		log.L.WithError(err).Warnf("could not parse %s: %s", disableEnvName, value)
-		return path
-	}
-
-	if disable {
-		return ""
-	}
-
 	return path
 }