Use the same key for the Reader build step across pipelines

See how
https://buildkite.com/automattic/wordpress-ios/builds/27417/steps?jid=01964b59-300d-46d2-bf40-33558caa03fa
failed because the pipeline used `reader_asc_build` instead of
`build_asc_reader`.

I decided to consolidate to `build_asc_reader` because of the
information hierarchy: build > build for ASC > build for ASC, the Reader
app.

When we'll get to split build from upload for the other apps, we'll be
able to have auto-complete friendly keys, su as `build_asc_wordpress` or
`build_firebase_jetpack`.

Author: mokagio

.buildkite/reader-testflight.yml

@@ -4,7 +4,7 @@
 steps:
   - label: ":eyeglasses: :xcode: Reader App Store Connect Build"
     command: .buildkite/commands/release-build-reader.sh
-    key: reader_asc_build
+    key: build_asc_reader
     plugins: [$CI_TOOLKIT_PLUGIN]
     agents:
       queue: mac
@@ -17,7 +17,7 @@ steps:
       - slack: "#build-and-ship"
 
   - label: ":eyeglasses: :testflight: Reader App Store Connect Upload"
-    depends_on: reader_asc_build
+    depends_on: build_asc_reader
     command: .buildkite/commands/release-upload-reader.sh
     plugins: [$CI_TOOLKIT_PLUGIN]
     agents: