Add experimental way to set a target platform when building

Add an --experimental-platform flag to the do command to allow
overriding the default auto-detected build platform until we find the
time to think about the definitive multi-platform builds UX

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

tests/plan/platform/platform.cue

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"dagger.io/dagger"
+	"dagger.io/dagger/core"
+)
+
+dagger.#Plan & {
+	actions: test: image: core.#Pull & {
+		source: "alpine:3.15.0"
+	}
+}