Update Nixpkgs version to 26.05

[aidenfoxivey]

As NixOS's underlying package repository Nixpkgs has been updated from version 25.05 to 26.05, the pin in liboqs's flake should be updated correspondingly.

https://nixos.org/blog/announcements/2026/nixos-2605/

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
  No.

• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged. Also, make sure to update the list of algorithms in the continuous benchmarking files: .github/workflows/kem-bench.yml and sig-bench.yml)
  No.

[aidenfoxivey]

nix build cooperates on my machine. I'm not sure I'm satisfied with just checking that a build completes, but actually running tests with the artifact generated in the flake may be superfluous work.

[aidenfoxivey]

Failure seems unrelated to PR:

--2026-06-05 20:26:10--  https://downloadmirror.intel.com/850782/sde-external-9.53.0-2025-03-16-lin.tar.xz
Resolving downloadmirror.intel.com (downloadmirror.intel.com)... 52.85.193.96, 52.85.193.78, 52.85.193.100, ...
Connecting to downloadmirror.intel.com (downloadmirror.intel.com)|52.85.193.96|:443... connected.
HTTP request sent, awaiting response... 202 Accepted
Length: 0 [text/html]
Saving to: 'sde.tar.xz'

     0K                                                        0.00 =0s

2026-06-05 20:26:10 (0.00 B/s) - 'sde.tar.xz' saved [0/0]

tar: This does not look like a tar archive
xz: (stdin): File format not recognized
tar: Child returned status 1
tar: Error is not recoverable: exiting now

[coveralls]

Coverage is 82.186% — aidenfoxivey:main into open-quantum-safe:main. No base build found for open-quantum-safe:main.

[baentsch]

LGTM. Failure due to apparent new restriction at Intel download sites --> If re-occurring, download Intel files, drop CI step, or re-design the test.

[aidenfoxivey]

Unrelated to this, but I have the S390X and PPC64LE self hosting CI PR in the works. I am sincerely sorry about the delay.