jenkins: use osmo-clean-workspace.sh before and after build

See osmo-ci change I2409b2928b4d7ebbd6c005097d4ad7337307dd93 for rationale.

Depends: I2409b2928b4d7ebbd6c005097d4ad7337307dd93
Change-Id: I99fdd20e7b18c2f4a59a9db35e5efccca6546b77
diff --git a/contrib/jenkins_common.sh b/contrib/jenkins_common.sh
index bc4ac65..a6ffe7f 100644
--- a/contrib/jenkins_common.sh
+++ b/contrib/jenkins_common.sh
@@ -8,6 +8,8 @@
     exit 1
 fi
 
+osmo-clean-workspace.sh
+
 verify_value_string_arrays_are_terminated.py $(find . -name "*.[hc]")
 
 prep_build() {
@@ -16,11 +18,8 @@
 
     cd "$_src_dir"
 
-    # a failed 'make distcheck' may leave files without write permissions
-    chmod -R a+w .
-    git clean -dxf
-    # make absolutely sure no src files have modifications
-    git checkout -f HEAD
+    # clean again before each build variant
+    osmo-clean-workspace.sh
 
     autoreconf --install --force