Run the FMI jobs in the .CI/testing image (#383)

The FMI stages passed '' as dockerfile, the old default they had to fill in
to reach the fmiSimulators argument. Since #335 made .CI/testing the default,
'' means opting out of the container, so the FMI jobs kept building omc on the
node. master-fmi now fails there: OpenModelica#16864 makes cmake use the system
minizip on Linux, and the node has no unzip.h.

Assisted-by: Claude Opus 5.5

Author: sjoelund

.CI/Jenkinsfile

@@ -217,7 +217,7 @@ pipeline {
           expression { params.fmi_v1_26 }
         }
         steps {
-          runRegressiontest('maintenance/v1.26', 'v1.26-fmi', '', omsimulatorHash(), false, '', '', false, false, 0, 'configs/conf.json', '', '', fmiSimulators())
+          runRegressiontest('maintenance/v1.26', 'v1.26-fmi', '', omsimulatorHash(), false, '', '', false, false, 0, 'configs/conf.json', '', '.CI/testing', fmiSimulators())
         }
       }
       stage('v1.27 FMI') {
@@ -233,7 +233,7 @@ pipeline {
           expression { params.fmi_v1_27 }
         }
         steps {
-          runRegressiontest('maintenance/v1.27', 'v1.27-fmi', '', omsimulatorHash(), false, '', '', false, false, 0, 'configs/conf.json', '', '', fmiSimulators())
+          runRegressiontest('maintenance/v1.27', 'v1.27-fmi', '', omsimulatorHash(), false, '', '', false, false, 0, 'configs/conf.json', '', '.CI/testing', fmiSimulators())
         }
       }
       stage('master FMI') {
@@ -249,7 +249,7 @@ pipeline {
           expression { params.fmi_master }
         }
         steps {
-          runRegressiontest('master', 'master-fmi', '', 'origin/master', false, '', '', false, false, 0, 'configs/conf.json', '', '', fmiSimulators())
+          runRegressiontest('master', 'master-fmi', '', 'origin/master', false, '', '', false, false, 0, 'configs/conf.json', '', '.CI/testing', fmiSimulators())
         }
       }