[FlINK-36392] fix path to MAVEN_HOME

TIL M2_HOME is deprecated/unsupported...

Author: SamBarker

Dockerfile

@@ -19,13 +19,13 @@
 ARG JAVA_VERSION=11
 FROM maven:3.8.8-eclipse-temurin-${JAVA_VERSION} AS build
 ARG SKIP_TESTS=true
-ARG M2_HOME=${HOME}/m2
+ARG MAVEN_HOME=${HOME}/.m2
 
 WORKDIR /app
 
 COPY . .
 
-RUN --mount=type=bind,source=${M2_HOME},target=/root/.m2 \
+RUN --mount=type=bind,source=${MAVEN_HOME},target=/root/.m2 \
      mvn -ntp clean install \
     -pl flink-kubernetes-standalone,flink-kubernetes-operator-api,flink-kubernetes-operator,flink-autoscaler,flink-kubernetes-webhook -DskipTests=$SKIP_TESTS