Fix memcpy alias bug in std.compress.lzma

[Szwagi]

Replaces memcpy with mem.copyForwards to prevent overlap panic.

thread 18216 panic: @memcpy arguments alias
C:\Program Files\Zig\lib\std\compress\lzma.zig:80:53: 0x18c72d in read (lzma.exe.obj)
            @memcpy(input[0 .. input.len - n], input[n..]);

The lzma tests never triggered the bug, I assume the inputs are too small so readAllAlloc never calls .read() for less than half of the buffer length, so if needed I attach some compressed input that does (let me know if I should get a minimal input that reproduces the issue and add a test).

test "lzma big file" {
    const allocator = std.testing.allocator;
    
    const compressed_file = try std.fs.cwd().openFile("compressed.txt", .{});
    defer compressed_file.close();

    var decompressor = try std.compress.lzma.decompress(allocator, compressed_file.reader());
    defer decompressor.deinit();
    
    var buffer = [1]u8{0};
    _ = try decompressor.read(buffer[0..]);
}

compressed.txt
expected.txt