Composer: avoid writing a lock file

[fredden]

Description

When working with this repository as a developer, we should be using the latest compatible packages. By writing a lock file for Composer, we may get into a state where we are "stuck" on an older version of a dependency. This change avoids such a situation by telling Composer to not write out a lock file in the project.

Suggested changelog entry

None required

Types of changes

• Chore - internal maintenance task with no impact on external dependencies nor consumers of the project.

PR checklist

• [x] I have checked there is no other PR open for the same change.
• [x] I have read the Contribution Guidelines.
• [x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• [x] I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.