env: a NUL byte in a `--file` entry reaches `std::env::set_var` unvalidated and aborts the process (exit 134)

env --file FILE parses an .env/INI file and passes each key and value straight to std::env::set_var, which panics when either contains a NUL byte. A single NUL anywhere in the file therefore aborts env with a Rust panic instead of a diagnostic.

The argv path (env NAME=VALUE ...) is guarded — apply_specified_env_vars checks for an empty name before calling set_var, but the --file path has no guard at all.

Steps to reproduce

A NUL in the value:

$ printf 'KEY=a\x00b\n' > conf.env
$ env --file conf.env
thread 'main' panicked at library/std/src/env.rs:361:9:
failed to set environment variable `"KEY"` to `"a\0b"`: file name contained an unexpected NUL byte
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
$ echo $?
134

A NUL in the key does the same:

$ printf 'a\x00b=c\n' > k.env
$ env --file k.env       # exit 134, identical panic

Root cause

https://github.com/uutils/coreutils/blob/2c0e1b981fdce56bb8cc729dffed702ff0410e8e/src/uu/env/src/env.rs#L331-L338

std::env::set_var panics if either argument contains a NUL. Nothing between the parser and this call validates the bytes.

Start in src/uu/env/src/env.rs around lines 331-338 and trace the --file path from parsed entries to std::env::set_var; compare it with apply_specified_env_vars, which guards the argv path. Reproduce the key and value NUL cases from the issue, then verify that both produce a diagnostic and non-panicking failure rather than exit 134.