Use `BoolishValueParser` for boolean environment variables (#3113)
## Summary
Right now, we only accept _exactly `UV_NATIVE_TLS=true` and
`UV_NATIVE_TLS=false`. `BoolishValueParser` accepts a wider range of
values:
```rust
/// True values are `y`, `yes`, `t`, `true`, `on`, and `1`.
pub(crate) const TRUE_LITERALS: [&str; 6] = ["y", "yes", "t", "true", "on", "1"];
/// False values are `n`, `no`, `f`, `false`, `off`, and `0`.
pub(crate) const FALSE_LITERALS: [&str; 6] = ["n", "no", "f", "false", "off", "0"];
```
I tend to use `0` and `1` personally so this surprised me.
