Instance executor: the "use_common_build_dir has no effect" warning is never shown

Summary

In executors/instance/instance.go, Prepare() logs the warning "use_common_build_dir has no effect: requires vm isolation or max_use_count = 1" through e.BuildLogger. At that point e.BuildLogger has not been set yet, because it is set by e.AbstractExecutor.Prepare(options), which is called a few lines later. The warning is dropped and the user is not told that use_common_build_dir is ignored.

Steps to reproduce

1. Configure an instance executor runner (any fleeting plugin) with:

[runners.instance]
  use_common_build_dir = true
[runners.autoscaler]
  max_use_count = 3        # > 1, and vm_isolation not enabled

2. Run any job.

Actual behavior

The job succeeds; the job log contains no warning.

Expected behavior

The job log contains:

WARNING: use_common_build_dir has no effect: requires vm isolation or max_use_count = 1

Used GitLab Runner version

19.4.0 (code unchanged on main, 19.6.0)

Possible fixes

Log through the build logger of the options (tested: the warning appears in the job log, the job succeeds):

--- a/executors/instance/instance.go
+++ b/executors/instance/instance.go
 		} else {
-			e.BuildLogger.Warningln("use_common_build_dir has no effect: requires vm isolation or max_use_count = 1")
+			options.BuildLogger.Warningln("use_common_build_dir has no effect: requires vm isolation or max_use_count = 1")
 		}