[CI] Add `shellcheck` and fix up warnings #205

Job	Run time
Soundness / API breakage check	2m 29s
Soundness / Broken symlinks check	4s
Soundness / Documentation check	2m 0s
Soundness / Format check	54s
Soundness / License headers check	8s
Soundness / Unacceptable language check	4s
Checks / Integration Tests / 5.8	1m 31s
Soundness / Shell check	7s
Checks / Integration Tests / 5.9	2m 37s
Checks / Integration Tests / 5.10	2m 42s
Checks / Integration Tests / nightly-6.0	3m 38s
Checks / Integration Tests / nightly-main	3m 28s
Checks / Benchmarks / 5.8	2m 43s
Checks / Benchmarks / 5.9	3m 54s
Checks / Benchmarks / 5.10	3m 36s
Checks / Benchmarks / nightly-6.0	4m 31s
Checks / Benchmarks / nightly-main	5m 15s
Checks / Cxx interop / 5.8	1m 1s
Checks / Cxx interop / 5.9	1m 11s
Checks / Cxx interop / 5.10	1m 12s
Checks / Cxx interop / nightly-6.0	1m 44s
Checks / Cxx interop / nightly-main	1m 41s
Checks / Unit tests / 5.8	3m 12s
Checks / Unit tests / 5.9	3m 22s
Checks / Unit tests / 5.10	3m 30s
Checks / Unit tests / nightly-6.0	4m 15s
Checks / Unit tests / nightly-main	4m 17s
	1h 5m 6s

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