Allow overriding AGENT_PACKAGE_VERSION and MANIFEST_URL when USE_PACKAGE_VERSION=true (#9864)

* Allow overriding AGENT_PACKAGE_VERSION and MANIFEST_URL when USE_PACKAGE_VERSION=true

* Update dev-tools/mage/settings.go

* fix lint errors

Author: pchila

dev-tools/mage/settings.go

@@ -162,17 +162,18 @@ func initGlobals() {
 
 	versionQualifier, versionQualified = os.LookupEnv("VERSION_QUALIFIER")
 
-	agentPackageVersion = EnvOr(agentPackageVersionEnvVar, "")
-
-	ManifestURL = EnvOr(ManifestUrlEnvVar, "")
-	PackagingFromManifest = ManifestURL != ""
-
-	// order matters this must be called last as it will override some of the
-	// values above
+	// order matters: this will override some of the values. Those values can be used
+	// as fallback for the variables below (mainly agentPackageVersion and ManifestURL)
 	err = initPackageVersion()
 	if err != nil {
 		panic(fmt.Errorf("failed to init package version: %w", err))
 	}
+
+	agentPackageVersion = EnvOr(agentPackageVersionEnvVar, agentPackageVersion)
+
+	ManifestURL = EnvOr(ManifestUrlEnvVar, ManifestURL)
+	PackagingFromManifest = ManifestURL != ""
+
 }
 
 // ProjectType specifies the type of project (OSS vs X-Pack).