Make stale_cachefile compatible with Nix mtime

[Sacha0]

The added condition improves compatibility with Nix mtime. Best! :)

[vtjnash]

I wonder if we could stop recording include'd files times if their read-only bits are set, possibly saving us some stat calls

[Sacha0]

Incidentally, in the last couple days I tested running without this patch (on a much more recent Nix build than the patch was originally written against), and our test suite appears to succeed without out. So it may be safe to drop this patch 😄.

[Sacha0]

So it may be safe to drop this patch 😄.

A little update: Our Nix guru (@rbvermaa) thinks that other Nix users will nonetheless benefit from inclusion of this patch, so it seems worthwhile :).

[DilumAluthge]

I wonder if we could stop recording include'd files times if their read-only bits are set, possibly saving us some stat calls

I don't think this is safe. I've run into situations in various CI environments where, even if you set a file to be read-only, you can still modify the file later in the CI job.

To be safe, I don't think we should assume that a read-only file will not be modified later.