only use half the cores for building/testing PETSc, since in particular the test suite can be quite memory-hungry

[boegel]

In EESSI/software-layer#1608 we saw the PETSc test step failing due to out-of-memory, so let's try using just half the available cores for building & testing PETSc...

Other than making the installation process a bit slower perhaps, there should be no harm in doing this across all PETSc versions.

[boegel]

bot: build repo:eessi.io-2023.06-software instance:eessi-bot-jsc for:arch=aarch64/nvidia/grace
bot: build repo:eessi.io-2025.06-software instance:eessi-bot-jsc for:arch=aarch64/nvidia/grace
bot: build repo:eessi.io-2026.06-software instance:eessi-bot-jsc for:arch=aarch64/nvidia/grace

[eessi-bot-jsc]

New job on instance eessi-bot-jsc for repository eessi.io-2023.06-software
Building on: nvidia-grace
Building for: aarch64/nvidia/grace
Job dir: /p/project1/ceasybuilders/eessibot/jobs/2026.09/pr_298/15588131

date	job status	comment
Sep 03 15:19:22 UTC 2026	submitted	job id 15588131 awaits release by job manager
Sep 03 15:20:25 UTC 2026	released	job awaits launch by Slurm scheduler

[eessi-bot-jsc]

New job on instance eessi-bot-jsc for repository eessi.io-2025.06-software
Building on: nvidia-grace
Building for: aarch64/nvidia/grace
Job dir: /p/project1/ceasybuilders/eessibot/jobs/2026.09/pr_298/15588132

date	job status	comment
Sep 03 15:19:29 UTC 2026	submitted	job id 15588132 awaits release by job manager
Sep 03 15:20:21 UTC 2026	released	job awaits launch by Slurm scheduler

[eessi-bot-jsc]

New job on instance eessi-bot-jsc for repository eessi.io-2026.06-software
Building on: nvidia-grace
Building for: aarch64/nvidia/grace
Job dir: /p/project1/ceasybuilders/eessibot/jobs/2026.09/pr_298/15588133

date	job status	comment
Sep 03 15:19:36 UTC 2026	submitted	job id 15588133 awaits release by job manager
Sep 03 15:20:16 UTC 2026	released	job awaits launch by Slurm scheduler