refactor: remove unreachable nil checks for ExportToCache

The determineDockerExportMode() function always sets cfg.ExportToCache
to a non-nil value before these conditions are evaluated, making the
nil checks unreachable dead code.

Simplified:
- Line 1958: (cfg.ExportToCache == nil || !*cfg.ExportToCache) → !*cfg.ExportToCache
- Line 2015: cfg.ExportToCache != nil && *cfg.ExportToCache → *cfg.ExportToCache

Co-authored-by: Ona <no-reply@ona.com>

Author: leodido

pkg/leeway/build.go

@@ -1955,7 +1955,7 @@ func (p *Package) buildDocker(buildctx *buildContext, wd, result string) (res *p
 		))
 
 		commands[PackageBuildPhasePackage] = pkgcmds
-	} else if len(cfg.Image) > 0 && (cfg.ExportToCache == nil || !*cfg.ExportToCache) {
+	} else if len(cfg.Image) > 0 && !*cfg.ExportToCache {
 		// Image push workflow
 		log.WithField("images", cfg.Image).Debug("configuring image push (legacy behavior)")
 
@@ -2012,7 +2012,7 @@ func (p *Package) buildDocker(buildctx *buildContext, wd, result string) (res *p
 
 		// Add subjects function for provenance generation
 		res.Subjects = createDockerSubjectsFunction(version, cfg)
-	} else if len(cfg.Image) > 0 && cfg.ExportToCache != nil && *cfg.ExportToCache {
+	} else if len(cfg.Image) > 0 && *cfg.ExportToCache {
 		// Export to cache for signing
 		log.WithField("package", p.FullName()).Debug("Exporting Docker image to cache")