.buildkite: temporary fix to manually install imgtool

Force CI to manually install imgtool, which is needed
for sanitycheck to build and run the TF-M integration
samples with MCUboot (where MCUboot is built from the
TF-M external project).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
diff --git a/.buildkite/run.sh b/.buildkite/run.sh
index b8c8f6f..2f4c073 100755
--- a/.buildkite/run.sh
+++ b/.buildkite/run.sh
@@ -53,6 +53,10 @@
 # lpcxpresso11u68 boards
 pip3 install lpc_checksum
 
+# Temporary fix: Install imgtool, needed for MCUboot to sign images
+# when builindg the TF-M integration samples
+pip3 install imgtool
+
 if [ -n "${DAILY_BUILD}" ]; then
    SANITYCHECK_OPTIONS=" --inline-logs -N --build-only --all --retry-failed 3 -v "
    echo "--- DAILY BUILD"