Apply shellcheck fixes

[nwiltsie]

Shellcheck is a tool for finding problematic code in shell scripts, from syntax errors to subtle and non-intuitive pitfalls. It is also capable of automatically fixed some kinds of issues.

Eventually shellcheck will be enabled in the CI/CD checks (i.e. the linters) used by this repository. In order to ease that transition (and make your life easier!), this PR includes all code fixes that could be performed automatically.

None of these fixes have been tested - this pull request was generated by a script and should be reviewed very closely.

These changes make the shell scripts in this repository abide by the following shellcheck rules:

• Double quote to prevent globbing and word splitting.

There were also warnings that could not be fixed automatically:

In modify_reference_path.sh line 6:
cat "${refpath_default}" | \
    ^------------------^ SC2002 (style): Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead.

For more information:
  https://www.shellcheck.net/wiki/SC2002 -- Useless cat. Consider 'cmd < file...

If any of these fixes/warnings are incorrect, shellcheck can be silenced by adding a line like #shellcheck disable=<rule name> immediately before the offending line (docs).

Also, you can just sidestep this issue completely by adding a .cicd-env file to the root of this repository with VALIDATE_SHELL="false" (reference). Please only do that as a last resort - linters are on your side!

[Faizal-Eeman]

Looks good!