Add a couple of missing spack --color=never

[albestro]

After spack/spack#42334 stackinator started raising some errors due to colorised output.

Just for reference, the error look like this

env: ‘\033[1m/user-environment/linux-sles15-zen/gcc-7.5.0/squashfs-4.6.1-pl5475bjii2xaf3cafaflr2yidf5754h\033[0m/bin/mksquashfs’: No such file or directory

In other places spack --color=never is already used, so I just applied the same solution also in the other parts that were triggering the error.

This applies for sure to develop branch, but in principle it should not harm in general.

Actually, I think it might be a good idea to have a different $(SPACK) variable that already embeds the --color=never, so it can be safely used where we use spack not as build tool but as helper to retrieve information (e.g. get install prefix).

[bcumming]

Thanks @albestro.

I agree about replacing SPACK with SPACK_HELPER.

[albestro]

@bcumming is it the way you were thinking about it?