std.posix.getenv: early-return comparison

[g-logunov]

Addresses the issue described in #22917.

Possibly interferes with @andrewrk work on https://github.com/ziglang/zig/tree/main branch.

Original implementation https://github.com/ziglang/zig/blob/aa3db7cc15/lib/std/posix.zig#L2004 for each environment variable iterates until the end of its name (until =), and only then compares entire name to key.
Since some of the environment variables could be quite long (i.e. GHOSTTY_SHELL_INTEGRATION_NO_SUDO=1), these sizes add up.
Simply - in order to find a key in environ, it has to iterate over cumulative sizes of each env variable name before it.

Proposed implementation functionally does what strncmp would do: stops iterating and moves to the next variable on first character mismatch with key.

See the benchmarks below.
Disclaimer: this was tested on macOS, with a variation of #23264 fix applied.

// getenv-c.zig
const std = @import("std");

pub fn main() void {
    for (0..10_000_000) |_| {
        _ = std.mem.doNotOptimizeAway(std.c.getenv("FOOBAR"));
    }
}

// getenv-zig.zig
const std = @import("std");

pub fn main() void {
    for (0..10_000_000) |_| {
        _ = std.mem.doNotOptimizeAway(std.posix.getenv("FOOBAR"));
    }
}

> env | wc -l
      43
> hyperfine --warmup 3 ./getenv-c ./getenv-zig-aa3db7cc15 ./getenv-zig-speedup
Benchmark 1: ./getenv-c
  Time (mean ± σ):     294.1 ms ±   3.5 ms    [User: 293.1 ms, System: 0.8 ms]
  Range (min … max):   288.7 ms … 299.3 ms    10 runs

Benchmark 2: ./getenv-zig-aa3db7cc15
  Time (mean ± σ):      1.907 s ±  0.022 s    [User: 1.901 s, System: 0.005 s]
  Range (min … max):    1.881 s …  1.941 s    10 runs

Benchmark 3: ./getenv-zig-speedup
  Time (mean ± σ):     130.8 ms ±   0.5 ms    [User: 129.8 ms, System: 0.8 ms]
  Range (min … max):   129.7 ms … 131.7 ms    22 runs

Summary
  ./getenv-zig-speedup ran
    2.25 ± 0.03 times faster than ./getenv-c
   14.58 ± 0.17 times faster than ./getenv-zig-aa3db7cc15

To address the elephant in the room: this loop is duplicated, but I'm hesitant to refactor it in order to deduplicate because of
// TODO see https://github.com/ziglang/zig/issues/4524 preamble and ongoing work to fix it.