Improve tty error logging when buildkit vertex is unknown (#2188)

* Improve tty error logging when buildkit vertex is unknown

Creates a generic "system" group in the tty output which captures
buildkit events that report a non-dagger vertex name. This happens
currently when using core.#Dockerfile actions since Dagger delegates the
LLB generation to buildkit through it's frontend and we don't get
meaningful events that we can correlate from Dagger's side

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

plancontext/fs.go

@@ -12,12 +12,10 @@ import (
 	"go.dagger.io/dagger/pkg"
 )
 
-var (
-	fsIDPath = cue.MakePath(
-		cue.Str("$dagger"),
-		cue.Str("fs"),
-		cue.Hid("_id", pkg.DaggerPackage),
-	)
+var fsIDPath = cue.MakePath(
+	cue.Str("$dagger"),
+	cue.Str("fs"),
+	cue.Hid("_id", pkg.DaggerPackage),
 )
 
 func IsFSValue(v *compiler.Value) bool {