* Add 'extra_env' argument to Executable.__call__: this will be added
to the environment but does not affect whether the current
environment is reused. If 'env' is not set, then the current
environment is copied and the variables from 'extra_env' are added
to it.
* MakeExecutable can take a 'jobs_env' parameter that specifies the
name of an environment variable used to set the level of parallelism.
This is added to 'extra_env' (so does not affect whether the current
environment is reused).
* CMake-based Spack packages set 'jobs_env' when executing the 'test'
target for make and ninja (which does not use -j)
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