Fix #2896

[dcooper16]

This PR fixes #2896. This commit avoids modifying the ADDTL_CA_FILES environment variable, and instead substitutes spaces for commas whenever the variable is used.

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What is your pull request about?

• Bug fix
• Improvement
• New feature (adds functionality)
• Breaking change (bug fix, feature or improvement that would cause existing functionality to not work as expected)
• Typo fix
• Documentation update
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If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
• I've read CONTRIBUTING.md and Coding_Convention.md
• I have tested this fix or improvement against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order) and the change to CHANGELOG.md

[drwetter]

Hi @dcooper16 : the CI test on MacOS wasn't successful so far. Just restarted it, again.

If that persists, maybe it's better to rebase. There were changes today for one check as the external host which we check again blocks request sometimes.

[drwetter]

yeah, it worked.

Thanks again, @dcooper16 !