[CI] Add `shellcheck` and fix up warnings #202

Job	Run time
Checks / Integration Tests / 5.8	1m 34s
Checks / Integration Tests / 5.9	2m 40s
Checks / Integration Tests / 5.10	2m 36s
Checks / Integration Tests / nightly-6.0	3m 27s
Checks / Integration Tests / nightly-main	3m 35s
Soundness / API breakage check	2m 29s
Soundness / Documentation check	2m 0s
Soundness / Unacceptable language check	6s
Soundness / License headers check	7s
Soundness / Broken symlinks check	2s
Soundness / Format check	49s
Soundness / Shell check	31s
Checks / Benchmarks / 5.8	2m 43s
Checks / Benchmarks / 5.9	3m 48s
Checks / Benchmarks / 5.10	3m 36s
Checks / Benchmarks / nightly-6.0	5m 5s
Checks / Benchmarks / nightly-main	5m 24s
Checks / Cxx interop / 5.8	59s
Checks / Cxx interop / 5.9	1m 7s
Checks / Cxx interop / 5.10	1m 4s
Checks / Cxx interop / nightly-6.0	1m 38s
Checks / Cxx interop / nightly-main	1m 37s
Checks / Unit tests / 5.8	3m 3s
Checks / Unit tests / 5.9	1h 48m 55s
Checks / Unit tests / 5.10	3m 9s
Checks / Unit tests / nightly-6.0	4m 13s
Checks / Unit tests / nightly-main	4m 4s
	2h 50m 21s

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