buildscripts: always keep mvn artifacts (#4372)

After searching for "artifacts retention policy" in the
kokoro-users group, I learned that there's a default 90 day
retention policy already in place.

buildscripts/kokoro/macos.cfg

@@ -4,7 +4,7 @@
 build_file: "grpc-java/buildscripts/kokoro/unix.sh"
 timeout_mins: 45
 
-# We always build artifacts, but we only copy them here when MVN_ARTIFACTS is set in unix.sh
+# We always build mvn artifacts.
 action {
   define_artifacts {
     regex: ["**/mvn-artifacts/**"]

buildscripts/kokoro/unix.sh

@@ -3,7 +3,6 @@
 # This file is used for both Linux and MacOS builds.
 # To run locally:
 #  ./buildscripts/kokoro/unix.sh
-# Optionally set MVN_ARTIFACTS=true to retain artifacts
 
 # This script assumes `set -e`. Removing it may lead to undefined behavior.
 set -exu -o pipefail
@@ -74,9 +73,6 @@ LOCAL_MVN_TEMP=$(mktemp -d)
 ./gradlew clean grpc-compiler:build grpc-compiler:uploadArchives $GRADLE_FLAGS -PtargetArch=x86_64 \
   -Dorg.gradle.parallel=false -PrepositoryDir=$LOCAL_MVN_TEMP
 
-if [[ -z "${MVN_ARTIFACTS:-}" ]]; then
-  exit 0
-fi
 MVN_ARTIFACT_DIR="$PWD/mvn-artifacts"
 mkdir $MVN_ARTIFACT_DIR
 mv $LOCAL_MVN_TEMP/* $MVN_ARTIFACT_DIR

buildscripts/kokoro/windows.bat

@@ -20,8 +20,6 @@ set PATH=C:\Program Files\java\jdk1.8.0_152\bin;%PATH%
 cmd.exe /C "%WORKSPACE%\buildscripts\kokoro\windows32.bat" || exit /b 1
 cmd.exe /C "%WORKSPACE%\buildscripts\kokoro\windows64.bat" || exit /b 1
 
-IF DEFINED MVN_ARTIFACTS (
-  mkdir mvn-artifacts
-  move artifacts\x86_64 mvn-artifacts\x86_64
-  move artifacts\x86_32 mvn-artifacts\x86_32
-)
+mkdir mvn-artifacts
+move artifacts\x86_64 mvn-artifacts\x86_64
+move artifacts\x86_32 mvn-artifacts\x86_32

buildscripts/kokoro/windows.cfg

@@ -3,6 +3,8 @@
 # Location of the continuous shell script in repository.
 build_file: "grpc-java/buildscripts/kokoro/windows.bat"
 timeout_mins: 45
+
+# We always build mvn artifacts.
 action {
   define_artifacts {
     regex: ["**/build/test-results/**/*.xml", "**/mvn-artifacts/**"]