Accessing Android app secrets from GitHub Actions using Gradle (https://blog.jakelee.co.uk/accessing-android-app-secret-from-github-actions-using-gradle/)

build.gradle

android {
    ....
    defaultConfig {
        ....
        buildConfigField ("String", "AUTH_CODE", getApiKey())
    }
}

String getApiKey() {
    def propFile = rootProject.file("./local.properties")
    def properties = new Properties()
    properties.load(new FileInputStream(propFile))
    return properties['APOD_API_KEY']
}

dependencies {
    ....
}

build.yml

name: CI
on:
  push:
    branches: [ develop ]
  pull_request:
    branches: [ develop ]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout the code
        uses: actions/checkout@v2
      - name: set up JDK 1.8
        uses: actions/setup-java@v1
        with:
          java-version: 1.8
      - name: Access APOD_API_KEY
        env:
          APOD_API_KEY: ${{ secrets.APOD_API_KEY }}
        run: echo APOD_API_KEY=\"$APOD_API_KEY\" > ./local.properties
      - name: Build the app
        run: ./gradlew assembleDebug

local.properties

## This file is automatically generated by Android Studio.
# Do not modify this file -- YOUR CHANGES WILL BE ERASED!
#
# This file should *NOT* be checked into Version Control Systems,
# as it contains information specific to your local configuration.
#
# Location of the SDK. This is only used by Gradle.
# For customization when using a Version Control System, please read the
# header note.
sdk.dir=C\:\\Users\\Jake\\AppData\\Local\\Android\\sdk
APOD_API_KEY="abcd1234"

MyClass.kt

package uk.co.jakelee.apodwallpaper.example

import uk.co.jakelee.apodwallpaper.BuildConfig

class MyClass() {
  val key = BuildConfig.APOD_API_KEY
}