ci: .shippable.yml: disable parallel build

We get random build failures with Shippable CI. First kind is [1]:

 /bin/bash: out/arm-plat-imx/conf.mk.tmp: No such file or directory
 core/core.mk:69: recipe for target 'out/arm-plat-imx/conf.mk' failed
 make: *** [out/arm-plat-imx/conf.mk] Error 1
 make: *** Waiting for unfinished jobs....

Second kind is [2]:

 mkdir: cannot create directory 'out/arm-plat-ti/core': No such file or directory
 mk/subdir.mk:151: recipe for target 'out/arm-plat-ti/core/ta_pub_key.c' failed
 make: *** [out/arm-plat-ti/core/ta_pub_key.c] Error 1
 make: *** Waiting for unfinished jobs....

Sometimes both bash and mkdir fail with 'No such file or directory' [3]

I cannot see any mistake in our Makefile or anything that could explain
these errors. I have opened a support request with Shippable [4].

In the mean time, let's disable the parallel build (make -jN) since I
could not reproduce the issue with a purely sequential build. Hopefully
it will not slow things down too much (we already need a reasonably
up-to-date cache anyway).

Link: [1] https://app.shippable.com/github/OP-TEE/optee_os/runs/347/1/console
Link: [2] https://app.shippable.com/github/OP-TEE/optee_os/runs/332/1/console
Link: [3] https://app.shippable.com/github/OP-TEE/optee_os/runs/342/1/console
Link: [4] https://github.com/Shippable/support/issues 3953
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>

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