fix(nix): Make sure bssl is in the PATH; workaround nix build failure…

[dougch]

Issues:

none

Description of changes:

In some CI environments (in docker containers), the build is trying to write to $HOME, which nix intentionally unsets. This causes build failures with this error:

FAILED: crypto/err_data.c /tmp/nix-build-aws-lc.drv-0/source/build/crypto/err_data.c
cd /tmp/nix-build-aws-lc.drv-0/source/crypto/err && /nix/store/2xc5jh4nxvm68c67ila9p2rj3qrjgz0x-go-1.23.5/bin/go run err_data_generate.go > /tmp/nix-build-aws->
failed to initialize build cache at /homeless-shelter/.cache/go-build: mkdir /homeless-shelter: permission denied

By setting $HOME in preBuild, we can eliminate this failure.

Also, the bssl utility is not available in a devShell, because we're not properly importing aws-lc.

Call-outs:

The build failure is blocking s2n-tls from using the upstream flake.

Reformatting both nix files with nix fmt.

Testing:

How is this change tested (unit tests, fuzz tests, etc.)? locally.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.88%. Comparing base (4e97131) to head (547db09).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2431      +/-   ##
==========================================
- Coverage   78.89%   78.88%   -0.01%     
==========================================
  Files         621      621              
  Lines      108613   108613              
  Branches    15414    15414              
==========================================
- Hits        85685    85681       -4     
- Misses      22258    22263       +5     
+ Partials      670      669       -1     

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