x-pack/dockerlogbeat: fix mage build failing on cross-arch platform (#48957)

Build() was missing the platform filter that Package() relied on,
causing it to attempt an arm64 cross-build on amd64 hosts (and vice
versa). Rename isSupportedPlatform to filterPlatformsForNativeArch to
make the side-effect explicit, and call it from Build() as well.

Author: orestisfl

x-pack/dockerlogbeat/magefile.go

@@ -360,6 +360,10 @@ func CrossBuild() error {
 
 // Build builds the base container used by the docker plugin
 func Build() {
+	if !filterPlatformsForNativeArch() {
+		fmt.Println(">> build: skipping because no supported platform is enabled")
+		return
+	}
 	mg.SerialDeps(CrossBuild, BuildContainer)
 }
 
@@ -381,7 +385,7 @@ func Package() {
 	start := time.Now()
 	defer func() { fmt.Println("package ran for", time.Since(start)) }()
 
-	if !isSupportedPlatform() {
+	if !filterPlatformsForNativeArch() {
 		fmt.Println(">> package: skipping because no supported platform is enabled")
 		return
 	}
@@ -397,7 +401,9 @@ func Ironbank() error {
 	return nil
 }
 
-func isSupportedPlatform() bool {
+// filterPlatformsForNativeArch removes cross-architecture platforms that
+// can't be built on the current host, and reports whether any remain.
+func filterPlatformsForNativeArch() bool {
 	_, isAMD64Selected := devtools.Platforms.Get("linux/amd64")
 	_, isARM64Selected := devtools.Platforms.Get("linux/arm64")
 	arch := runtime.GOARCH