Fix plugin upgrade process

[tmpfs]

When updating the minimum required dependency for a plugin several errors happen.

First we get a not satisfied error which is solved by deleting the lock file. Then we get a package digest mismatch error which is solved by clearing the downloads cache (upm clean downloads).

To test this bump a plugin version in site.toml for a project that has a site.lock file that is up to date.

Just tested going from std::core@^5 to std::core@^5.0.1.