fix(ci): keep the Busser root outside the checkout

The runners require "bundler/setup", which walks up from the suite looking for
a Gemfile. With the Busser root under ${{ github.workspace }} it found this
project's Gemfile and tried to materialize that bundle inside the isolated gem
home, which cannot work:

```text
Could not find cookstyle-9.0.0, aruba-2.4.1, cucumber-11.1.1, ...
  in locally installed gems (Bundler::GemNotFound)
```

A real Busser root is /opt/busser -- never inside the project -- so this was an
artefact of the CI layout rather than anything the plugins do. Moving it to
/tmp/busser-kitchen matches reality and fixes it.

It only showed up on CI because a root outside any Gemfile tree, which is what I
had locally, makes bundler/setup a no-op.

Signed-off-by: Tim Smith <tsmith84@proton.me>

Author: tas50

.github/workflows/integration.yml

@@ -26,7 +26,12 @@ jobs:
         # integration, not Ruby compatibility.
         ruby: ["3.2", "4.0"]
     env:
-      BUSSER_ROOT: ${{ github.workspace }}/tmp/busser-kitchen
+      # Outside the checkout on purpose. The runners require "bundler/setup",
+      # which walks up from the suite looking for a Gemfile -- so a Busser root
+      # inside the workspace finds this project's Gemfile and tries to
+      # materialize that bundle inside the isolated gem home, which fails. A
+      # real Busser root is /opt/busser, never inside the project.
+      BUSSER_ROOT: /tmp/busser-kitchen
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1