Add SOONG_DISABLE_CLIPPY environment variable

This environment variable will be used for testing the Rust toolchain
for correctness before updating the Android codebase to resolve new
lints.

Test: Introduce lint
Test: SOONG_DISABLE_CLIPPY=true m out/soong/.intermediates/frameworks/minikin/rust/libminikin_rust_ffi/android_arm_armv8-a_rlib_rlib-std/libminikin_rust_ffi.rlib.clippy
Change-Id: If7236c7f16a3e0a0b27a90577211549f2dc1a344