fix: Add COVERAGE_RCFILE to accept customized .coveragerc

[nlou9]

Revisit this closed issue, #1217.
We still face the same issue. Inspired by the comments in the issue thread, we add a COVERAGE_RCFILE environment variable to be able to pass the customized .coveragerc file.

In our own project, we patched this way, and it resolved our issue.

py_test(
    name = "test_utils",
    srcs = ["test_utils.py"],
    data = ["//:.coveragerc"],
    env = {
        "COVERAGE_RCFILE": "$(location //:.coveragerc)",
    },
    deps = [
        ":conftest",
        "//tests/data",
    ],
)