Use slightly fewer threads for pika in CI

[msimberg]

Requires pika 0.32.0 for the PIKA_THREADS environment variable to have an effect.

[msimberg]

cscs-ci run

[msimberg]

Leaving one core free for other threads in this PR also speeds up the C API tests quite a bit (in addition to #1268). Setting PIKA_THREADS requires pika 0.32.0, so it won't apply to all CI configurations (we want to keep one on the oldest supported version 0.30.X), but we could use 0.32.0 on the CUDA configurations where there are more tests to run.

[RMeli]

Setting PIKA_THREADS requires pika 0.32.0, so it won't apply to all CI configurations

Ugly, but can't you still use the workaround PIKA_COMMANDLINE_OPTIONS="--pika:threads=..." which works with earlier versions too?

[msimberg]

Setting PIKA_THREADS requires pika 0.32.0, so it won't apply to all CI configurations

Ugly, but can't you still use the workaround `PIKA_COMMANDLINE_OPTIONS="--pika:threads=..." which works with earlier versions too?

In theory that should've been possible, but the real command line options and those supplied by PIKA_COMMANDLINE_OPTIONS do not mix well... (overriding each other, but not quite).