fix a warning about impure expression

[satorg]

See details in Discord

Under some circumstances the build can start failing with the following error:

[error] .../ghostdogpr/caliban/codegen-sbt/src/main/scala/caliban/codegen/CompileTimeCalibanPlugin.scala:269:57: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
[error]                           ensureCompiled(serverProject).value
[error]                                                         ^
[error] No warnings can be incurred under -Xfatal-warnings.
[error] two errors found
[error] (codegenSbt / Compile / compileIncremental) Compilation failed

Usually it does not happen, however can show up if one uses Metals to work with the project (e.g. vscode).

[ghostdogpr]

Thanks!