Do not add -sysroot flag when ANDROID_NDK_ROOT environment is set

[marcprux]

When the environment ANDROID_NDK_ROOT is set, the driver manually adds it as a --sysroot flag. This breaks building with an Android SDK when this variable is set, with confusing errors. I first saw this at finagolfin/swift-android-sdk#207 (comment), and more recently ran into it again (https://github.com/swift-android-sdk/swift-docker/actions/runs/14584558227/job/40907674425) while working on the official Android SDK build script, where it fails a build with:

<unknown>:0: error: missing required module 'SwiftAndroid'

In both cases, manually un-setting ANDROID_NDK_ROOT (which is set by default in GitHub workflows) resolved the issue.

This flag has some history (#1560, #1681, and #1811), but I feel like it should just be excised altogether since the Android SDK can specify its own sdkRootPath in the swift-sdk.json file. If Windows needs it due to lack of Swift SDK support, then it could be gated inside an #if os(Windows) check.

And BTW, the #if arch(x86_64) check is incorrect: the Android NDK works on both Intel and ARM macOS machines, despite being stored under "darwin-x86_64", as the binaries are universal:

zap ~ % file $ANDROID_HOME/ndk/27.0.12077973/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang
~/Library/Android/sdk/ndk/27.0.12077973/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64]
~/Library/Android/sdk/ndk/27.0.12077973/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang (for architecture x86_64):	Mach-O 64-bit executable x86_64
~/Library/Android/sdk/ndk/27.0.12077973/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang (for architecture arm64):	Mach-O 64-bit executable arm64