Make run-functional-tests script more robust

[agateau-gg]

Context

While debugging a CI failure, I tried to run functional tests using make functest and it failed. This PR fixes that.

What has been done

• Do not fail if the build directory exists
• Do not fail (at least on my machine) when listing the wheel
• Remove the [all] when installing the wheel: there is no all variant anymore

Validation

Run make functest locally, with a build dir.