Make packer cache branches explicit (elastic#41990)

Before this change we would recurse to cache bwc versions.
This proved to be problematic due to  the number of steps it was
generating taking too long.
Also this required tricky maintenance to break the recursion for old
branches we don't really care about.

With this change we now cache specific branches only.

.ci/packer_cache.sh

@@ -22,3 +22,4 @@ export JAVA8_HOME="${HOME}"/.java/java8
 export JAVA11_HOME="${HOME}"/.java/java11
 export JAVA12_HOME="${HOME}"/.java/openjdk12
 ./gradlew --parallel clean --scan -Porg.elasticsearch.acceptScanTOS=true -s resolveAllDependencies
+

distribution/bwc/build.gradle

@@ -239,12 +239,15 @@ bwcVersions.forPreviousUnreleased { BwcVersions.UnreleasedVersionInfo unreleased
 
         createBuildBwcTask(projectName, "${baseDir}/${projectName}", projectArtifact)
     }
-
 
     createRunBwcGradleTask("resolveAllBwcDependencies") {
         args 'resolveAllDependencies'
     }
-    resolveAllDependencies.dependsOn resolveAllBwcDependencies
+    Version currentVersion = Version.fromString(version)
+    if (currentVersion.getMinor() == 0 && currentVersion.getRevision() == 0) {
+        // We only want to resolve dependencies for live versions of master, without cascading this to older versions
+        resolveAllDependencies.dependsOn resolveAllBwcDependencies
+    }
 
     for (e in artifactFiles) {
         String projectName = e.key