[Bug 2132941] [NEW] Rust unwrap with UTF-8 symbols in environment variables

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In Ubuntu 25.10, if an environment variable or its value contains a UTF-8 character, coreutils will unwrap in Rust's library/std/src/env.rs.
Expected behavior is either to handle the UTF-8 character like any other character or to print an error message.

Simple reproducer (tested with Bash):
  STR=$(echo -e "foo\xA0")
  # Generate environment variable 'foo' with UTF-8 value
  foo=$STR env | grep foo
  # Generate environment variable with UTF-8 name
  declare "$STR=bar" ; env | grep $STR

The second command triggers this unwrap message in Ubuntu 25.10, but works fine on Fedora 43:
  thread 'main' panicked at library/std/src/env.rs:163:83:
  called `Result::unwrap()` on an `Err` value: "foo\xA0"
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The third command fails with "declare: `foo�=bar': not a valid identifier" (same on Ubuntu 25.10 and Fedora 43).
Using `printf -v` shows the same error message.
This behavior might be fine, but it can be bypassed by e.g. Perl:
  #!/usr/bin/perl
  $str = "foo\x{A0}";
  $ENV{$str} = 'bar';
  use open IN => ":raw";
  my @env = `env`;
  chomp (my @env = grep { s/^$str=// } @env);
  die("@env not equal to 'bar'") if "@env" ne "bar"

This Perl script executes fine on Fedora 43, but triggers the following error in Ubuntu 25.10:
  thread 'main' panicked at library/std/src/env.rs:163:57:
  called `Result::unwrap()` on an `Err` value: "foo\xA0"
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

** Affects: rust-coreutils (Ubuntu)
     Importance: Undecided
         Status: New

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Rust unwrap with UTF-8 symbols in environment variables
https://bugs.launchpad.net/bugs/2132941
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