CI-unixish.yml: fail `build` step if unversioned files exist at the end

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This is to expose tests (or other commands) which generate (and leave) files in the source tree. Files which are being ignored should not fail this.

This should primarily be fixed by using temporary folders. In case this can't be done we need to make sure they are cleaned up - and in some cases we might also need to clean previously existing files beforehand.

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Okay, it succeeding was unexpected. Maybe these stray files are (incorrectly) covered by .gitignore - will investigate.

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HEAD detached at pull/6787/merge
Untracked files:
  (use "git add <file>..." to include in what will be committed)
	.ccache/
	addons/test/cppcheck-addon-ctu-file-list
	cmake.output.signal/
	cppcheck-addon-ctu-file-list
	test-signalhandler
	test-stacktrace
	test/cli/cppcheck-addon-ctu-file-list
	test/cli/helloworld/cppcheck-addon-ctu-file-list
	test/cli/helloworld/test.cppcheck
	test/cli/proj2/test.cppcheck
	test/cli/test.cppcheck
nothing added to commit but untracked files present (use "git add" to track)