ensure-tox: fix pipe race

The

 command -v pip pip3 | head -n1

introduced with Ie50928c9b782ea84db916bb1441567e1206ff466 has a very
subtle race; if "pip" and "pip3" exists and there are two lines
output, the "head -n1" will exit and depending on scheduling the
"command" might write to a broken pipe (this manifests as exit code
141).

Move this to a more explicit if statement.

Co-Authored-By: Jens Harbott <j.harbott@x-ion.de>
Change-Id: I80823a7bc6351925d6f0b20bdebca3eafef0b27d

roles/ensure-tox/tasks/main.yaml

@@ -1,7 +1,13 @@
 - name: Ensure tox is installed
   shell: |
     set -euo pipefail
-    PIP=$(command -v pip pip3 | head -n1)
+
+    if command -v pip; then
+      PIP=pip
+    elif command -v pip3; then
+      PIP=pip3
+    fi
+
     type tox || $PIP install --user tox
   args:
     executable: /bin/bash