Small fixes on the cmake element

[aleixpol]

• Fix cmake double builds, implements it as it was workarounded already on the freedesktop-sdk
• Use cmake env CMAKE_BUILD_PARALLEL_LEVEL to tell the amount of jobs to execute at once rather than passing the argument to ninja/make

[abderrahim]

The reason the workaround was implemented in freedesktop-sdk rather than here is that it would be a breaking change: it would change the cache keys, which is a stability promise of this package.

It would be nice to figure out a better way to implement such changes in a backwards compatible way. Either as an option, or as versioned plugins that projects could migrate towards.

[aleixpol]

#92 was fixed by @juergbi on a different layer, still I believe there's value in this change.

I think there's still value here as it's simplifying and effectively using the cmake calls as they're meant to be used.

Furthermore, addressing the comment by @abderrahim would be useful as having an effective "no changes allowed" policy in the project is dangerous for the project's health as it forces users to workaround it rather than to submit fixes upstream. I started a conversation here and in case it's something we can/want to address: https://lists.apache.org/thread/0z1679z3v9twl8yxgcyw94rsrz4l9roc