fix regressions in _prefix_pwd_to_flag in cargo_build_script.bzl

[KirillTim]

also extract _pwd_flags_* functions to a list to improve readability.

Tested:

bazel test //cargo/tests/cargo_build_script/cc_args_and_env:all

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[KirillTim]

While working on #4216, I noticed some regressions in the way we handle compiler flags. These aren't just theoretical corner cases; they can easily happen in standard use cases, so I decided to fix them here.