Get fallback MPI completion mode from runtime configuration

[msimberg]

This fixes the PIKA_MPI_COMPLETION_MODE environment variable not being taken into account.

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
:white_check_mark: -0.10% (target: -1.00%)	:white_check_mark: 100.00% (target: 90.00%)

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (ea9120e076e415dd1280f35977489b6e1afc038f) | 18368 | 13724 | 74.72% | | | Head commit (a35a5a003eb8386e0ef0628b4bd53456df8af847) | 18370 (+2) | 13708 (-16) | 74.62% (**-0.10%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#1215) | 5 | 5 | **100.00%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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