tests: added test to ensure an op.#Exec with no cache will not execute twice

Signed-off-by: Sam Alba <sam.alba@gmail.com>

tests/compute/success/exec-nocache/main.cue

@@ -0,0 +1,25 @@
+package main
+
+import (
+    "dagger.io/alpine"
+    "dagger.io/dagger/op"
+)
+
+rand: {
+    string
+
+    #up: [
+        op.#Load & {from: alpine.#Image},
+        op.#Exec & {
+                always: true
+				args: ["sh", "-c", """
+					tr -dc A-Za-z0-9 </dev/urandom | head -c 13 > /id
+					"""]
+        },
+        op.#Export & {source: "/id"},
+    ]
+}
+
+// If rand is executed twice, cue won't validate
+ref1: rand
+ref2: ref1