`labgrid_test` should respect environment variables set in targets passed to `src`

In the example below, GREETING isn't available inside hello.py:

py_binary(
    name = "inner",
    srcs = ["hello.py"],
    env = {
        "GREETING": "hello",
    },
)

labgrid_test(
    name = "test",
    src = ":inner",
    platform = "@rules_labgrid//labgrid/platform:localhost",
)