[CI] Add `shellcheck` and fix up warnings #201

Job	Run time
Soundness / API breakage check	1m 37s
Soundness / Documentation check	1m 35s
Soundness / Broken symlinks check	2s
Checks / Integration Tests / 5.8	1m 36s
Checks / Integration Tests / 5.9	1m 36s
Soundness / Format check	51s
Soundness / License headers check	8s
Checks / Integration Tests / 5.10	1m 34s
Soundness / Shell check	29s
Checks / Integration Tests / nightly-6.0	1m 37s
Soundness / Unacceptable language check	5s
Checks / Integration Tests / nightly-main	1m 36s
Checks / Benchmarks / 5.8	1m 35s
Checks / Benchmarks / 5.9	1m 35s
Checks / Cxx interop / 5.8	57s
Checks / Benchmarks / 5.10	1m 35s
Checks / Cxx interop / 5.9	1m 10s
Checks / Benchmarks / nightly-6.0	1m 34s
Checks / Cxx interop / 5.10	1m 6s
Checks / Benchmarks / nightly-main	1m 33s
Checks / Cxx interop / nightly-6.0	1m 36s
Checks / Cxx interop / nightly-main	1m 35s
Checks / Unit tests / 5.8	1m 34s
Checks / Unit tests / 5.9	1m 34s
Checks / Unit tests / 5.10	1m 36s
Checks / Unit tests / nightly-6.0	1m 34s
Checks / Unit tests / nightly-main	1m 34s
	34m 54s

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