Renamed _build_script_run rule to cargo_build_script

[UebelAndre]

Today the primary target of a cargo_build_script entry in a BUILD file is actually a _build_script_run rule. This makes targets hard to query for folks who are not intimately aware of how this functionality is implemented. To reduce confusion for users, the rule seen in their BUILD files should be something they can query for. For example:

rules_rust % bazel query 'kind(cargo_build_script, //...)'
//test/build_env:cargo_build_script_env-vars_build_script
//test/cargo_build_script:tools_exec_build_rs
//test/dep_env:read_a
//test/dep_env:read_b
//test/dep_env:read_dep_dir
//test/dep_env:set_a_build
//test/out_dir_in_tests:build_script
//test/transitive_lib:buildscript
//test/unit/transitive_link_search_paths:dep_build_script
//test/unit/transitive_link_search_paths:proc_macro_build_script
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[illicitonion]

Makes sense to me - thanks!