Skip linux_bionic by default in the build tests.

It doesn't work in the build test - it's for use in Soong-only builds.
Specifically Soong fails for dependencies on libLLVM_android, which is
disabled in those builds. To work it needs either
FORCE_BUILD_LLVM_COMPONENTS=true or
SOONG_ALLOW_MISSING_DEPENDENCIES=true to be passed to the build.

Test: build_test presubmits
Bug: 31559095
Bug: 229123823
Change-Id: I7a2b231816012de01472b478e7a98e2b0e929363