Fix #4644: Set custom protobuf artifact for macOS

[adhiamboperes]

Explanation

Fix #4644:

Execution failed for task ':model:generateProto'.

Could not resolve all files for configuration ':model:protobufToolsLocator_protoc'.
Could not find protoc-3.8.0-osx-aarch_64.exe (com.google.protobuf:protoc:3.8.0).
Searched in the following locations:
https://repo.maven.apache.org/maven2/com/google/protobuf/protoc/3.8.0/protoc-3.8.0-osx-aarch_64.exe`

Changed the conditional check for the protobuf_platform property from checking for property presence to checking for the current os. This is because project.rootProject.hasProperty('protobuf_platform') does not actually read the properties file as it is meant to, therefore the if condition was never met and the artifact was never set correctly.

I tested the existing implementaion by adding in build.gradle:
println project.rootProject.property('protobuf_platform')
This throws an error:
Caused by: groovy.lang.MissingPropertyException: Could not get unknown property 'protobuf_platform' for root project 'oppia-android' of type org.gradle.api.Project.
Which shows that the current solution for reading the property from local.properties file does not work correctly.

This solution changes how the property is read by loading the local.properties file with inputstream.

Essential Checklist

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[BenHenning]

Nice catch! I just had one comment, but otherwise need to see CI fully passing before this can be merged. PTAL @adhiamboperes at the new comment.

[adhiamboperes]

Nice catch! I just had one comment, but otherwise need to see CI fully passing before this can be merged. PTAL @adhiamboperes at the new comment.

I added a close operation to the inputStream. PTAL @BenHenning at the new commit.

[BenHenning]

Thanks @adhiamboperes! This LGTM.