Fix builder inspect showing known builders as untrusted

[runesoerensen]

Summary

builder_inspect.go was missing the IsKnownTrustedBuilder check that other commands already include, causing known trusted builders to always show as untrusted.

• Add IsKnownTrustedBuilder check in builder_inspect.go, matching the pattern in build.go and config_trusted_builder.go

Output

Before

$ pack builder inspect heroku/builder:24
...
Trusted: No

After

$ pack builder inspect heroku/builder:24
...
Trusted: Yes

Documentation

• Should this change be documented?
  • No

Related

Resolves #2570