.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>
1 file changed