Use a sysroot for Linux host builds by default (#359)
Use a sysroot for Linux host builds by default (#359)

If no explicit sysroot is set, and it's not disabled, use a sysroot for
Linux host builds. This means we will no longer require specific
libraries to be on CI images for host builds, and developers won't need
as much local setup.

Fixes https://github.com/flutter/flutter/issues/53176
flutter/buildroot,flutter/buildroot,flutter/buildroot,flutter/buildroot