Passing absolute path to a directory from genrule in a Bazel BUILD file

Question

I'm trying to set an environment variable for my mocha_test rule in a Bazel BUILD file to provide the absolute path to a directory that's created using a genrule. The genrule is responsible for creating the directory and processing some dependencies, but I'm not sure how to pass the correct absolute path using "location".

Here's my current BUILD file:

genrule(
    name = "app_dir_rule",
    srcs = [],
    outs = ["app_dir"],
    cmd = """
        mkdir -p $@

        $(location :app_binary) process_dependencies --output=$@/vendor
        cp $(location :app_binary) $@
    """,
    tools = [":app_binary"],
    visibility = ["//visibility:private"],
)

mocha_test(
    name = "mocha",
    args = ["--inspect"],
    chdir = package_name(),
    data = [
        ":.mocharc.js",
        ":.prettierignore",
        ...
        ":app_dir_rule"
    ],
    env = {
        "APP_DIR": "$(location :app_dir_rule)"
    },
    tags = ["local"],
)

Could anyone help me to figure out the correct way to pass the absolute path for my application directory in this Bazel BUILD file??