Introduce shellcheck to lint shell scripts

[straight-shoota]

This PR fixes a number of issues in several shell scripts that shellcheck complains about. It also introduces a new CI job to ensure shellcheck compliance in the future.

Issues fixed:

• https://www.shellcheck.net/wiki/SC2145
• https://www.shellcheck.net/wiki/SC2027
• https://www.shellcheck.net/wiki/SC2034
• https://www.shellcheck.net/wiki/SC2046
• https://www.shellcheck.net/wiki/SC2124
• https://www.shellcheck.net/wiki/SC2164
• https://www.shellcheck.net/wiki/SC2294
• https://www.shellcheck.net/wiki/SC2059
• https://www.shellcheck.net/wiki/SC2086
• https://www.shellcheck.net/wiki/SC2295
• https://www.shellcheck.net/wiki/SC2002
• https://www.shellcheck.net/wiki/SC2005
• https://www.shellcheck.net/wiki/SC2006

Issues ignored:

• https://www.shellcheck.net/wiki/SC2013

Follow-up to #15139

[oprypin]

It will be nice to have this 🙂