Expose more configuration options as environment variables

[msimberg]

This exposes:

• PIKA_IGNORE_PROCESS_MASK
• PIKA_THREADS
• PIKA_SCHEDULER
• PIKA_BIND
  as environment variables that can be used to set a default for the corresponding configuration options. PIKA_SCHEDULER corresponds to pika.scheduler and --pika:queueing. --pika:queuing has also been renamed to --pika:scheduler here.

Adds tests for the most important variables: PIKA_THREADS, PIKA_PROCESS_MASK, PIKA_IGNORE_PROCESS_MASK, and PIKA_BIND.

The following are also exposed in this PR as environment variables, but I don't intend to document them as I feel like they could be removed any time. They're exposed for development purposes only.

• PIKA_AFFINITY
• PIKA_PU_STEP
• PIKA_PU_OFFSET
• PIKA_NUMA_SENSITIVE

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.30% (target: -1.00%)	✅ 93.75% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (4694a13)	18217	13772	75.60%
Head commit (d932bdd)	18223 (+6)	13721 (-51)	75.29% (-0.30%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1380)	32	30	93.75%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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