[chore](build) Add MVN_OPT env variable to enrich building FE with extra options (apache#28375)

e.g. just export or add it to custom_env.sh
```
export MVN_OPT="-o"
```
will build FE with maven option "-o" (offline), which means maven does
not need to download meta from maven repo, it is useful for saving time
if the internet is unstable or unusable.

Author: gavinchou

build.sh

@@ -590,9 +590,9 @@ if [[ "${FE_MODULES}" != '' ]]; then
         clean_fe
     fi
     if [[ "${DISABLE_JAVA_CHECK_STYLE}" = "ON" ]]; then
-        "${MVN_CMD}" package -pl ${FE_MODULES:+${FE_MODULES}} -Dskip.doc=true -DskipTests -Dcheckstyle.skip=true
+        "${MVN_CMD}" package -pl ${FE_MODULES:+${FE_MODULES}} -Dskip.doc=true -DskipTests -Dcheckstyle.skip=true ${MVN_OPT:+${MVN_OPT}}
     else
-        "${MVN_CMD}" package -pl ${FE_MODULES:+${FE_MODULES}} -Dskip.doc=true -DskipTests
+        "${MVN_CMD}" package -pl ${FE_MODULES:+${FE_MODULES}} -Dskip.doc=true -DskipTests ${MVN_OPT:+${MVN_OPT}}
     fi
     cd "${DORIS_HOME}"
 fi