ci(mvp-smoke): install just before invoking the mvp-env recipe

`MVP Smoke (Best Effort)` has been failing with exit code 127 on every
push:

  /home/runner/work/_temp/…/sh: line 1: just: command not found
  ##[error]Process completed with exit code 127.

The workflow's final step is `just --justfile Justfile mvp-env`, but the
`ubuntu-latest` runner image does not include `just` and no step
installs it. Adding the canonical SHA-pinned `taiki-e/install-action`
between `Cache Cargo` and `Build release` (also runtime-relevant for
the smoke recipe) restores green.

SHA pin (`184183c… # v2.78.1`) matches the estate convention already
used across hypatia (ci.yml, security.yml, tests.yml,
build-gossamer-gui.yml). Tool selector `tool: just` is the standard
single-package form.

Effect: `MVP Smoke (Best Effort)` returns success on this branch and on
every subsequent push to `main`.

Refs: noted as one of the orthogonal failures on PR #128's checks
during the 2026-05-30 dependabot-deadlock triage.

Author: hyperpolymath

.github/workflows/mvp-smoke.yml

@@ -37,6 +37,11 @@ jobs:
       - name: Cache Cargo
         uses: Swatinem/rust-cache@65012b490220f477f20ab979e35ae732e6de4e68 # v2
 
+      - name: Install just
+        uses: taiki-e/install-action@184183c2401be73c3bf42c2e61268aa5855379c1 # v2.78.1
+        with:
+          tool: just
+
       - name: Build release
         run: cargo build --release