Fix frontend test fixture

We can no longer reference other sources. Instead, we can specify a
`local` source with the current directory in order to build the
frontend.

Signed-off-by: Peter Engelbert <pmengelbert@gmail.com>

test/fixtures/frontend.yml

@@ -10,6 +10,9 @@ vendor: Microsoft
 packager: Microsoft <support@microsoft.com>
 license: Apache 2.0
 sources:
+  src:
+    local:
+      path: .
   gomodcache:
     path: /build/gomodcache
     image:
@@ -19,8 +22,8 @@ sources:
         mounts:
           - dest: /build/src
             spec:
-              context:
-                name: ""
+              local:
+                path: .
         steps:
           - command: go mod download
             env: