fix: Don't treat default env vals as explicit

[charliermarsh]

Summary

We frequently see user reports in uv (e.g., astral-sh/uv#13385) related to the fact that environment variables set to fasely values still report as conflicting. Here's a minimal example from #5591 (comment):

use clap::Parser;

#[derive(Debug, Parser)]
pub struct Args {
    /// Don't resolve.
    #[arg(long, env = "FROZEN", value_parser = clap::builder::BoolishValueParser::new(), conflicts_with_all = ["check"])]
    frozen: bool,

    /// Check that the resolution is valid.
    #[arg(long, env = "CHECK", value_parser = clap::builder::BoolishValueParser::new())]
    check: bool,
}

fn main() {
    let args = Args::parse();
    println!("Args: {args:?}");
}

$ CHECK=0 mini-uv --frozen
error: the argument '--frozen' cannot be used with '--check'

Usage: clap-conflicts --frozen

For more information, try '--help'.

This PR modifies the parsing to treat "set from environment" behavior as non-explicit if the value matches the default.

Closes #5591.

[epage]

@charliermarsh I appreciate the interest in moving this forward but this is not a change to be made lightly and the way forward is design discussions on the issue and not a PR, per our contrib guidelines