fix: Add executable to runfiles of tools

[blorente]

Otherwise, depending on the tool target is usually not enough to use the tool as part of the Bazel build.

For instance: I'm trying to use pkg-config to compile the alsa-sys crate. I want to point the PKG_CONFIG env var to a pkg-config managed by rules_multitool, so I do it through an annotation:

crate.annotation(
    build_script_env = {
        "PKG_CONFIG": "$(execpath @multitool//tools/pkg-config:pkg-config)",
    },
    build_script_tools = [ # Works really well with multitool, since it selects for the `exec` platform.
        "@multitool//tools/pkg-config:pkg-config",
    ],
    crate = "alsa-sys",
)

Before this change, only the symlink got materialized in the sandbox, so it (understandably) failed to run. Now, the symlink's target is also materialized.