test: skip example satisfiability checks on unsupported platforms

[tdejager]

Description

Because the new pytest runs every example workspace on every CI runner, but pixi tree defaults to the current platform.

  pixi tree --locked --no-install --manifest-path <example>

Some examples are intentionally platform-specific. For example:

  # examples/tasks/pixi.toml
  platforms = ["osx-arm64"]

So on Linux CI, the test runs:

  pixi tree --locked --no-install --manifest-path examples/tasks/pixi.toml

and pixi tree tries to use Linux by default. But the example only supports osx-arm64, so it fails with:

  no platform supported by environment 'default' matches the current system

Hopefully fixes CI failure found in: #6301 and #6302 and maybe other PR's

Tool used: Codex with Pi

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas