feat: Allow default values in client: env (#2122)

Signed-off-by: Helder Correia <174525+helderco@users.noreply.github.com>

tests/plan/client/env/usage.cue

@@ -9,6 +9,7 @@ dagger.#Plan & {
 	client: env: {
 		TEST_STRING: string
 		TEST_SECRET: dagger.#Secret
+		TEST_DEFAULT: string | *"hello world"
 	}
 	actions: {
 		image: core.#Pull & {
@@ -19,6 +20,10 @@ dagger.#Plan & {
 				input: image.output
 				args: ["test", client.env.TEST_STRING, "=", "foo"]
 			}
+			default: core.#Exec & {
+				input: image.output
+				args: ["test", client.env.TEST_DEFAULT, "=", "hello world"]
+			}
 			secret: core.#Exec & {
 				input: image.output
 				mounts: secret: {