ci: use flakybot with matrix workflows

[kweinmeister]

Refactored the 5 functions workflows that currently use a matrix strategy, to use a common workflow template that includes FlakyBot 1.2+.

Considerations:

• Using the matrix strategy has a slightly different syntax. It did not look possible to integrate these workflows into the standard template using the tools available in GH Actions today.
• Also, these workflows require manually specifying subfolders within the matrix. So, it is safer to handle these workflows separately when regenerating workflows.

Changes:

• Workflow generation
  • Creation of a new "matrix" workflow template to handle this unique structure: ci-matrix.yaml.njk
  • A workflows-matrix.json array of workflows using a matrix strategy was created. These workflows require the workflow developer to manually specify the subdirectories that need to be tested within them. So, they shouldn't be included along with all of the other workflows that can be safely automatically generated.
  • CLI option in generate.js to specify the matrix template.
• Workflow updates
  • All existing matrix workflows were updated to use the standardized workflow.
• Sample updates
  • functions/concepts samples are small gists that do not have tests. To enable using the reusable test.yaml, a no-op npm test script was added.

[kweinmeister]

FYI @averikitsch @pattishin, in case you can take a look. This rolls out #3079 to an initial set of samples, which were using a matrix strategy to parallelize testing across the samples' subfolders. Thanks!

[bcoe]

Left a few recommendations.

[kweinmeister]

Left a few recommendations.

Thanks for the very helpful suggestions!