PVF worker: random fixes (#7649)

* PVF worker: random fixes

- Fixes possible panic due to non-UTF-8 env vars
  (https://github.com/paritytech/polkadot/pull/7330#discussion_r1300101716)
- Very small refactor of some duplicated code

* Don't need `to_str()` for comparison between OsString and str

* Check edge cases that can cause env::remove_var to panic

In case of a key or value that would cause env::remove_var to panic, we first
log a warning and then proceed to attempt to remove the env var.

* Make warning message clearer for end users

* Backslash was unescaped, but can just remove it from error messages