ci: branch checkout and apk sign issues

[oldkingOK]

Hi, I encountered errors when trying to use GitHub Action to built the apk, which were resolved by making some fixes to build.yml.

• zju-connect no longer has an android branch, and the AAR can be built successfully without git checkout .
• an error when signing the apk.

Error: Couldnt find the Android build tools @ /usr/local/lib/android/sdk/build-tools/30.0.2
Error: Unable to locate executable file: /usr/local/lib/android/sdk/build-tools/30.0.2/zipalign. Please verify either the file path exists or the file can be found within a directory specified by the PATH environment variable. Also check the file mode to verify the file is executable.

After checking this issue, adding an environment variable fixed it.

[Mythologyli]

Thanks for your contribution. Cheers!