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.sh b/contrib/jenkins.sh
index d731504..5798c95 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -29,3 +29,5 @@
 # is well separated from the source tree state.
 $MAKE distcheck \
     || cat-testlogs.sh
+
+osmo-clean-workspace.sh