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

See osmo-ci change I2409b2928b4d7ebbd6c005097d4ad7337307dd93 for rationale.

Depends: I2409b2928b4d7ebbd6c005097d4ad7337307dd93
Change-Id: If5d252dc679e47a8375361d0ff278b7b98768f26
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index d4f98f3..5f31710 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -2,6 +2,7 @@
 # jenkins build helper script for libasn1c.  This is how we build on jenkins.osmocom.org
 
 set -e
+osmo-clean-workspace.sh
 
 set +x
 echo
@@ -16,3 +17,5 @@
 $MAKE $PARALLEL_MAKE
 $MAKE distcheck \
   || cat-testlogs.sh
+
+osmo-clean-workspace.sh