Specify nginx binary via NGINX_EXE env var

[DanGould]

Set NGINX_EXE env var in CI workflow

Summary

The ohttp-relay integration tests use the NGINX_EXE environment variable to
locate the nginx binary (introduced in payjoin#1237). However, the GitHub Actions CI
workflow was still using the old approach of adding nginx to $GITHUB_PATH
instead of setting NGINX_EXE. This meant the integration tests were silently
skipped in CI since NGINX_EXE was not set.

This PR updates both the Test and Coverage jobs in the CI workflow to set
NGINX_EXE via $GITHUB_ENV, pointing to the full path of the nginx binary
built from the nginx-with-stream nix package.

Changes

• .github/workflows/rust.yml: Replace $GITHUB_PATH with $GITHUB_ENV to
  set NGINX_EXE to the full binary path in both Test and Coverage jobs

Test plan

• CI Test job runs ohttp-relay integration tests (no longer skipped)
• CI Coverage job runs ohttp-relay integration tests (no longer skipped)

Closes payjoin#1227

Disclosure: co-authored by claude-code

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch session4/issue-1227

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coveralls]

Pull Request Test Coverage Report for Build 22572331925

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.8%) to 83.238%

---

Totals
Change from base Build 22572195695:	0.8%
Covered Lines:	10721
Relevant Lines:	12880

---

💛 - Coveralls