Respect ANDROID_SDK_ROOT and ANDROID_NDK_ROOT env vars to override the Android SDK and/or NDK locations

[jakepetroules]

Currently Swift Build discovers the Android SDK and NDK under the default paths on each platform:

• Windows: %LOCALAPPDATA%\Android\Sdk
• macOS: ~/Library/Android/sdk
• Linux: ~/Android/Sdk

If ANDROID_SDK_ROOT and/or ANDROID_NDK_ROOT are set in the environment, those should override the default SDK and NDK location lookup, respectively.

[marcprux]

Note that ANDROID_NDK_ROOT is currently consulted, but in a way that breaks Android builds, at least on macOS and Linux. We currently need to explicitly un-set the environment in order to build for Android.

If we could solve the long-standing swiftrt.o issue, then ideally we could eliminate the "ndk-root" link script that we're using (see swiftlang/swift-docker#467 (comment)) and just default the sdkRootPath (which probably ought to be optional, but isn't) to use the ANDROID_NDK_ROOT environment, and everything would just work.

[jakepetroules]

Note that ANDROID_NDK_ROOT is currently consulted, but in a way that swiftlang/swift-driver#1879, at least on macOS and Linux. We currently need to explicitly un-set the environment in order to build for Android.

You're talking about swift-driver, but this issue is about Swift Build. We still need to consult ANDROID_NDK_ROOT here at this level in order to look up various locations correctly. One reason is because Swift Build will explicitly pass --sysroot to the Swift driver regardless of whether ANDROID_NDK_ROOT is set.

It sounds like your #1879 PR will fix the swift-driver issue though, right? Shall we move that out of draft and get it landed?

[marcprux]

I've moved swiftlang/swift-driver#1879 out of draft