Ensure the user-provided mvnw wrapper script is executable (#133)

If a developer has accidentally changed the permissions on their `mvnw` executable they should be able to deploy.

GUS-W-9689448

Previously if the `mvnw` file was not executable this would error:


```
       [Installing Java]
       JDK 1.8 installed
       JRE 1.8 installed

       [Installing Maven]
       [INFO] Maven wrapper detected, skipping installation.

       [Executing Maven]
       [INFO] $ ./mvnw -DskipTests clean install


       ERROR: failed to build: exit status 1

       stderr: /cnb/buildpacks/heroku_maven/0.2.5/bin/build: line 188: ./mvnw: Permission denied
       [ERROR: Failed to build app with Maven]
       We're sorry this build is failing! If you can't find the issue in application code,
       please submit a ticket so we can help: https://help.heroku.com/
       ERROR: failed to build: executing lifecycle: failed with status code: 145
```

Co-authored-by: Ed Morley <[email protected]>

buildpacks/maven/CHANGELOG.md

@@ -3,10 +3,12 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Fixed
+* Ensures `mvnw` is executable
 
 ## [0.2.4] 2021/07/16
 ### Added
-* Loosen stack requiremets allowing any linux distro use this buildpacak
+* Loosen stack requiremets allowing any linux distro use this buildpack
 
 ## [0.2.3] 2021/05/05
 ### Added

buildpacks/maven/bin/build

@@ -114,6 +114,7 @@ fi
 maven_executable="mvn"
 if maven::should_use_wrapper_for_app "${app_dir}"; then
 	maven_executable="./mvnw"
+	chmod +x "${maven_executable}"
 fi
 
 maven_options=()

test/specs/maven/misc_spec.rb

@@ -1,6 +1,17 @@
 require_relative "spec_helper"
 
 describe "Heroku's Maven Cloud Native Buildpack" do
+  it "will ensure mvn is executable regardless of file permissions" do
+    Cutlass::App.new("simple-http-service").transaction do |app|
+      FileUtils.chmod(0444, app.tmpdir.join("mvnw")) # Set executable to read only on purpose
+
+      app.pack_build do |pack_result|
+        expect(pack_result.stdout).to include("Successfully built image")
+        expect(pack_result.success?).to be_truthy
+      end
+    end
+  end
+
   it "will write ${APP_DIR}/target/mvn-dependency-list.log with the app's dependencies" do
     Cutlass::App.new("simple-http-service", config: {MAVEN_CUSTOM_GOALS: "clean"}).transaction do |app|
       app.pack_build do |pack_result|