Remove a self-dependency of bp2build

bp2build (i.e. bootstrap.ninja#bp2build_workspace_marker) generate $OUT/soong/bp2build/**/BUILD.bazel files
Having them as a dependency would thus make bp2build_workspace_marker stale upon incremental builds (because we don't re-touch the marker file if it already exists)

See Also: https://android-review.googlesource.com/c/platform/build/soong/+/2263423

Bug: b/253450880
Test: NINJA_ARGS='-d explain' m --bazel-mode adbd
      repeat and see if bp2build is rerun
prior to this CL, one would see the input /usr/local/google/home/usta/aosp/out/soong/bp2build/build/make/tools/BUILD.bazel trigger bp2build over and over again

Change-Id: I904cd333a5d6ef506fc4019eda7623ef96a1daa3