Prevents files to be used as input of dagger.#FS

Errors out in the PreRun phase of the clientfilesystemreader task since
otherwise, dagger execution would hang
Fixes #1977

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

tests/plan/client/filesystem/read/fs/invalid/test.cue

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"dagger.io/dagger"
+)
+
+dagger.#Plan & {
+	client: filesystem: "../rootfs/test.txt": read: contents: dagger.#FS
+	actions: test: {
+	}
+}