Based on file content. Detects elf magic header or shebang line.
Executable bit is ignored in hash calculation, as it was before this. So
packages hashes are not changed.
Reference:
https://github.com/ziglang/zig/issues/17463#issuecomment-1984798880
Fixes: 17463
Test is here:
7c4600d7bb/src/main.zig (L307)
(if #19500 got accepted I'll move this test to the Fetch.zig)