CI: upstream Viceroy 0.16.6 requires rustc 1.95, exceeds repo's 1.91.1 pin

[aram356]

What's wrong

Upstream Viceroy published viceroy-lib@0.16.6, which bumped its MSRV to rustc 1.95. rust-toolchain.toml pins this repo to rustc 1.91.1, so any cargo install --git https://github.com/fastly/Viceroy viceroy (no tag) now fails at compile:

error: failed to compile `viceroy v0.16.6 (https://github.com/fastly/Viceroy#9900324b)`
Caused by:
  rustc 1.91.1 is not supported by the following package:
    viceroy-lib@0.16.6 requires rustc 1.95

Immediate fix

#656 pins .github/workflows/test.yml to --tag v0.16.4, matching the tag already used by .github/actions/setup-integration-test-env/action.yml. This unbreaks CI.

Follow-up this issue tracks

The pin is a workaround, not a resolution. We should:

1. Decide the ongoing strategy for keeping Viceroy current given the MSRV gap — either bump the repo's rustc pin to something Viceroy supports, or continue pinning Viceroy to a compatible tag and track upstream releases deliberately.
2. Consolidate the Viceroy version into a single source of truth so .github/workflows/test.yml and .github/actions/setup-integration-test-env/action.yml can't drift. (e.g., a repo variable, a .viceroy-version file read by both, or a composite action.)
3. Add a CI step or dependabot rule that flags when Viceroy ships a new tag that is incompatible with our rustc pin, so we catch this before the gap widens further.

References

• Failing run: see the CI log that triggered Pin Viceroy install to v0.16.4 in test workflow #656.
• rust-toolchain.toml — current rustc pin (1.91.1).
• .github/workflows/test.yml — the workflow that was pinning by moving HEAD.
• .github/actions/setup-integration-test-env/action.yml — already pinned to v0.16.4.