Rustc flags for cargo_bazel_bin

[dk333333]

After pulling in #1781, we're now running into error: bazel-out/.../zlib_archive/libzlib.a(crc32.o): requires unsupported dynamic reloc 11; recompile with -fPIC.

We do get a little further with relocation-model=static, but I'm wondering if there's a better way to set this?

rust_binary(
    name = "cargo_bazel_bin",
    srcs = ["src/main.rs"],
    edition = "2018",
    version = VERSION,
    visibility = ["//visibility:public"],
+   rustc_flags = ["-C", "relocation-model=static"],
    deps = [":cargo_bazel"],
)

However this doesn't get us to a working state as we hit:

Error: `cargo metadata` exited with an error: error: manifest path `/tmp/.tmpR6uXck` contains no package: The manifest is virtual, and the workspace has no members.