vpc construct: only try setting context if unset

currently a single stack cannot create multiple vpcs, as each will try
to set the context itself, and the subsequent creations will throw
errors like

  Cannot set context after children have been added: MyVpc

.changeset/dirty-cherries-hug.md

@@ -0,0 +1,5 @@
+---
+"@guardian/cdk": patch
+---
+
+Fix bug preventing creation of multiple VPCs in single stack

src/constructs/vpc/vpc.test.ts

@@ -11,4 +11,13 @@ describe("The GuVpc construct", () => {
     new GuVpc(stack, "MyVpc");
     expect(Template.fromStack(stack).toJSON()).toMatchSnapshot();
   });
+
+  it("should be possible to create two vpcs in the same stack", () => {
+    const stack = simpleGuStackForTesting({
+      stack: "test-stack",
+      env: { region: "eu-west-1", account: "000000000000" },
+    });
+    new GuVpc(stack, "MyVpc");
+    new GuVpc(stack, "MyOtherVpc");
+  });
 });

src/constructs/vpc/vpc.ts

@@ -61,11 +61,11 @@ export class GuVpc extends Vpc {
       );
     }
 
-    node.setContext(`availability-zones:account=${account}:region=eu-west-1`, [
-      "eu-west-1a",
-      "eu-west-1b",
-      "eu-west-1c",
-    ]);
+    const contextKey = `availability-zones:account=${account}:region=eu-west-1`;
+
+    if (node.tryGetContext(contextKey) === undefined) {
+      node.setContext(contextKey, ["eu-west-1a", "eu-west-1b", "eu-west-1c"]);
+    }
   }
 
   constructor(scope: GuStack, id: string, props?: GuVpcProps) {