Review dependency patterns collection in Jenkins (#18004)

Fix dependency checking for generators, looking now for dependencies
of the generators code itself (beatgen) and Metricbeat beater.
Stop looking for dependencies of libbeat in all beats, this is redundant
as they are already transitive dependencies.
Run collection of dependecies in the host instead of using docker, what
is slightly faster.

Jenkinsfile

@@ -40,13 +40,13 @@ pipeline {
       steps {
         deleteDir()
         gitCheckout(basedir: "${BASE_DIR}")
+        stash allowEmpty: true, name: 'source', useDefaultExcludes: false
         dir("${BASE_DIR}"){
           loadConfigEnvVars()
         }
         whenTrue(params.debug){
           dumpFilteredEnvironment()
         }
-        stash allowEmpty: true, name: 'source', useDefaultExcludes: false
       }
     }
     stage('Lint'){
@@ -853,7 +853,6 @@ def isChangedOSSCode(patterns) {
     "^\\.ci/.*",
   ]
   allPatterns.addAll(patterns)
-  allPatterns.addAll(getVendorPatterns('libbeat'))
   return isChanged(allPatterns)
 }
 
@@ -868,14 +867,17 @@ def isChangedXPackCode(patterns) {
     "^\\.ci/.*",
   ]
   allPatterns.addAll(patterns)
-  allPatterns.addAll(getVendorPatterns('x-pack/libbeat'))
   return isChanged(allPatterns)
 }
 
 def loadConfigEnvVars(){
   def empty = []
   env.GO_VERSION = readFile(".go-version").trim()
 
+  withEnv(["HOME=${env.WORKSPACE}"]) {
+    sh(label: "Install Go ${env.GO_VERSION}", script: ".ci/scripts/install-go.sh")
+  }
+
   // Libbeat is the core framework of Beats. It has no additional dependencies
   // on other projects in the Beats repository.
   env.BUILD_LIBBEAT = isChangedOSSCode(empty)
@@ -934,17 +936,25 @@ def loadConfigEnvVars(){
   // involved.
   env.BUILD_KUBERNETES = isChanged(["^deploy/kubernetes/.*"])
 
-  env.BUILD_GENERATOR = isChangedOSSCode(getVendorPatterns('generator'))
+  def generatorPatterns = ['^generator/.*']
+  generatorPatterns.addAll(getVendorPatterns('generator/common/beatgen'))
+  generatorPatterns.addAll(getVendorPatterns('metricbeat/beater'))
+  env.BUILD_GENERATOR = isChangedOSSCode(generatorPatterns)
 }
 
 /**
   This method grab the dependencies of a Go module and transform them on regexp
 */
 def getVendorPatterns(beatName){
+  def os = goos()
+  def goRoot = "${env.WORKSPACE}/.gvm/versions/go${GO_VERSION}.${os}.amd64"
   def output = ""
-  docker.image("golang:${GO_VERSION}").inside{
+
+  withEnv([
+    "HOME=${env.WORKSPACE}/${env.BASE_DIR}",
+    "PATH=${env.WORKSPACE}/bin:${goRoot}/bin:${env.PATH}",
+  ]) {
     output = sh(label: 'Get vendor dependency patterns', returnStdout: true, script: """
-      export HOME=${WORKSPACE}/${BASE_DIR}
       go list -mod=vendor -f '{{ .ImportPath }}{{ "\\n" }}{{ join .Deps "\\n" }}' ./${beatName}\
         |awk '{print \$1"/.*"}'\
         |sed -e "s#github.com/elastic/beats/v7/##g"