Remove the hard-coded path of build_verity_metadata.py.

build_image.py used to invoke build_verity_metadata.py with a hard-coded
path of 'system/extras/verity/build_verity_metadata.py', which makes it
hard to run unittests from non-$(ANDROID_BUILD_TOP) directory.

This CL adds the dependency on the tool, so that it gets installed to
$(HOST_OUT_EXECUTABLES), then removes the hard-coded path.

Bug: 74544459
Bug: 76015688
Test: `m dist`
Test: python -m unittest test_validate_target_files
Change-Id: I0dcf4eb067a0db6f099cb589eb99a151a05c7f2b
diff --git a/core/config.mk b/core/config.mk
index e72b55a..be8c9a4 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -677,6 +677,7 @@
 BUILD_IMAGE_SRCS := $(wildcard build/make/tools/releasetools/*.py)
 APPEND2SIMG := $(HOST_OUT_EXECUTABLES)/append2simg
 VERITY_SIGNER := $(HOST_OUT_EXECUTABLES)/verity_signer
+BUILD_VERITY_METADATA := $(HOST_OUT_EXECUTABLES)/build_verity_metadata.py
 BUILD_VERITY_TREE := $(HOST_OUT_EXECUTABLES)/build_verity_tree
 BOOT_SIGNER := $(HOST_OUT_EXECUTABLES)/boot_signer
 FUTILITY := $(HOST_OUT_EXECUTABLES)/futility-host