Minor: Add `RuntimeEnvBuilder::build_arc()

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

Small follow on to https://github.com/apache/datafusion/pull/12157 from @devanbenz

It is still somewhat awkward to make RuntimeEnv's, so let's make it slightly easier

What changes are included in this PR?

Adds a convenience method to RuntimeEnvBuilder to build an Arc<RuntimeEnv>

Are these changes tested?

Yes, by CI

Are there any user-facing changes?

A new method