fix: check if files are same in self-update

[apoorvkh]

self-update checks if the Pixi binary matches the default filepath. Instead of checking if paths are equal, we should check if files are same in self-update.

This should resolve this issue:

  × pixi is not installed in the default location:
  │ 
  │ - Default pixi location: /users/apoorvkh/.pixi/bin/pixi
  │ - Pixi location detected: /nfs/home/apoorvkh/.pixi/bin/pixi

which could be caused by using symlinks or case-insensitivity in Windows, etc.

Fixes #1171 and maybe #2059

[ruben-arts]

Thank you this looks good to me! What makes this a Draft PR?

[apoorvkh]

Draft PR because I just wanted to run the CI! I haven't yet manually tested it for the case in #1171 .

[apoorvkh]

Okay did some testing. I think this implies the fix in my PR works!

---

When I use (the official) pixi=0.30.0, I get the same error as before.

❯ pixi self-update
  × pixi is not installed in the default location:
...

But when I do pixi self-update --force, I get an error:

❯ pixi self-update --force
thread 'main' panicked at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/reqwest-0.12.5/src/async_impl/client.rs:1901:38:
Client::new(): reqwest::Error { kind: Builder, source: Os { code: 13, kind: PermissionDenied, message: "Permission denied" } }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

(This error does not occur when I use e.g. pixi=0.25.0.)

---

When I run pixi self-update from this PR's binary, I get that PermissionDenied error directly.
Since we don't see the "not installed in the default location" error message, I think our patch was successful.

I have no idea why there is a PermissionDenied error (my pixi binary has rwx r-x r-x permissions), but I think that is outside the scope of this immediate PR.

[ruben-arts]

Thank you @apoorvkh !