CI: drop bashism from template overrides

[[ is bash only

In many distributions /bin/sh == bash but not in Debian and derived
where it is dash (very simple POSIX shell).

Change-Id: I2b084ea78b236623b174473f411bae04e624a3e9
(cherry picked from commit f0c3567)

Author: hrw

tests/templates/template_overrides.j2

@@ -83,7 +83,7 @@ ENV PIP_TRUSTED_HOST=
 ENV PIP_EXTRA_INDEX_URL=
 {% endif %}
 
-RUN if [[ -f /usr/etc/npmrc ]]; then \
+RUN if [ -f /usr/etc/npmrc ]; then \
         unlink /usr/etc/npmrc; \
     fi \
     && rm -f /etc/npmrc