Managed FetchService should not start the fetch-service

[tigarmo]

Bug Description

The managed version of the FetchService, which is used when packing to create the project manifest, should not try to start the fetch-service from inside the instance. It's actually surprising that it doesn't fail every time, only sometimes.

To Reproduce

Run pack with --enable-fetch-service=permissive

part yaml

.

Relevant log output

❯ rockcraft pack --enable-fetch-service=permissive
Warning: the fetch-service integration is experimental and still in development.rehash: warning: skipping ca-certificates.crt,it does not contain exactly one certificate or CRL
Warning: the fetch-service integration is experimental and still in development. 
The 'fetch-service' snap is not installed.
Recommended resolution: Install the fetch-service snap via 'snap install --channel=beta fetch-service'.
Failed to execute rockcraft in instance.

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3601.

This message was autogenerated