Use distro pip on Ubuntu

Running get-pip.py fails on Ubuntu when running twice, e.g. after a
unstack/stack cycle. Just use distro pip instead.

Closes-Bug: #1957048
Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: I87a8d53ed8860dd017a6c826dee6b6f4baef3c96
diff --git a/tools/install_pip.sh b/tools/install_pip.sh
index 259375a..5d73a1f 100755
--- a/tools/install_pip.sh
+++ b/tools/install_pip.sh
@@ -129,6 +129,8 @@
     # For general sanity, we just use the packaged pip.  It should be
     # recent enough anyway.  This is included via rpms/general
     : # Simply fall through
+elif is_ubuntu; then
+    : # pip on Ubuntu 20.04 is new enough, too
 else
     install_get_pip
 fi