Maven M2_Home应该是什么？

• 我安装了Maven sudo apt install maven
• 然后，从此仓库运行mvn clean install。
• 我收到以下错误

[ERROR] org.eclipse.xtext.maven.plugin.XtextGeneratorIT  Time elapsed: 4.246 s  <<< FAILURE!
java.lang.AssertionError: 
Maven home not found. Tried to call 'mvn'.
Consider to set the envVar 'M2_HOME' or System property 'maven.home'.
Current settings are: maven.home=null M2_HOME=null

我应该M2_HOME是什么？？？我输入mvn --version了一个线索：

$ mvn --version
Apache Maven 3.6.0
Maven home: /usr/share/maven
Java version: 11.0.7, vendor: Ubuntu, runtime: /usr/lib/jvm/java-11-openjdk-amd64
Default locale: en_IL, platform encoding: UTF-8
OS name: "linux", version: "4.15.0-109-generic", arch: "amd64", family: "unix"

但是我不确定现在该怎么办。任何帮助表示赞赏，谢谢！