[AppCheck] Fix 'public_headers_dir' in 'FirebaseAppCheckInterop.podspec'

[ncooke3]

I'm not sure why the CI in #11860 didn't surface this.

I locally tried to stage the FirebaseAppCheckInterop.podspec after merging #11860 and saw:

Validating spec
 -> FirebaseAppCheckInterop (10.17.0)
    - NOTE  | url: The URL (https://twitter.com/Firebase) is not reachable.
🔥    - WARN  | file patterns: The `public_header_files` pattern did not match any file.
    - NOTE  | xcodebuild:  note: Using codesigning identity override: -
    - NOTE  | xcodebuild:  note: Building targets in dependency order
    - NOTE  | [iOS] xcodebuild:  /var/folders/hz/7v68p4h95sb2ypb614xp20jc00r7sc/T/CocoaPods-Lint-20231011-82337-1s67496-FirebaseAppCheckInterop/App.xcodeproj: warning: The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 10.0, but the range of supported deployment target versions is 11.0 to 16.1.99. (in target 'App' from project 'App')
    - NOTE  | [iOS] xcodebuild:  Pods.xcodeproj: warning: The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 10.0, but the range of supported deployment target versions is 11.0 to 16.1.99. (in target 'FirebaseAppCheckInterop' from project 'Pods')
    - NOTE  | [iOS] xcodebuild:  Pods.xcodeproj: warning: The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 10.0, but the range of supported deployment target versions is 11.0 to 16.1.99. (in target 'Pods-App' from project 'Pods')
    - NOTE  | xcodebuild:  note: Using codesigning identity override: 

[!] The `FirebaseAppCheckInterop.podspec` specification does not validate.

#no-changelog

[paulb777]

Good catch! Did the new client app find it?

[ncooke3]

@paulb777, nope– it never had a chance. I ran into it when I tried to manually stage the pod after it merged to master. Let me see if it would have though...

[paulb777]

Looks like we're missing pod lib lint CI for the interop podspecs

[ncooke3]

Will open a PR to add linting for the interop pods.

And, the client app caught it (log). I rebased #11914 on top of master and enabled the client_app / cocoapods workflow.