Update to NixOS 26.05

[elliottslaughter]

General notes:

• Relies on Update to NixOS 26.05 lockshaw/proj#17: once that PR is merged, we can revert the proj URL to upstream
• Pinned cuDNN to preserve compatibility with NVIDIA Pascal GPUs
• Added a patch for nixGL based on (not-yet-merged) Update nixGL for latest nixpkgs: nvidia kernel modules and package deprecations nix-community/nixGL#223 for compatibility with NixOS 26.05
• Modern Nix is pickier about Python dependencies so we need to be more explicit about any vendored packages now

About the source changes specifically:

• Extra headers are not included transitively by GCC 14, so we need to be more explicit now
• Several changes were made for Doxygen compatibility: e.g., using C = ... was causing Doxygen warnings, and the current build errors on any Doxygen warnings at all. We could potentially work around this differently (e.g., not making warnings a hard error)
• For some reason we seem to have lost the num_gpus=2 in the Realm GPU model parallelism test, so I put that back here

Passes local CPU and GPU CI on Sapling.

I should note: this also includes #1659 so you can merge that one separately or as a part of this PR.

---

This change is 

[elliottslaughter]

I should note: we could potentially expand the deploy tests to sweep a larger set of GCC versions now (since we've fixed compatibility with newer versions), but I'm not sure how much we care. We should keep the current pinned versions since that's what we actually deploy on Sapling, but if we move to other machines in the future likely we'll care about newer versions as well.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.25%. Comparing base (03e4e07) to head (5433364).

Files with missing lines	Patch %	Lines
lib/op-attrs/src/op-attrs/parallel_dim.cc	0.00%	2 Missing ⚠️
...alm-execution/test/src/realm-execution/test_e2e.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1660       +/-   ##
===========================================
+ Coverage        0   71.25%   +71.25%     
===========================================
  Files           0     1544     +1544     
  Lines           0    53819    +53819     
===========================================
+ Hits            0    38348    +38348     
- Misses          0    15471    +15471     

Flag	Coverage Δ
unittests	71.25% <25.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/utils/include/utils/containers/filter.h	57.14% <ø> (ø)
...ib/utils/include/utils/containers/inplace_filter.h	0.00% <ø> (ø)
.../utils/include/utils/containers/unordered_set_of.h	28.57% <100.00%> (ø)
...alm-execution/test/src/realm-execution/test_e2e.cc	72.09% <0.00%> (ø)
lib/op-attrs/src/op-attrs/parallel_dim.cc	0.00% <0.00%> (ø)

... and 1539 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.