pathogen-repo-ci: Add input for GH Action runner

[joverlee521]

Based on @victorlin's proposed solution in nextstrain/conda-base#83

In nextstrain/.github/.github/workflows/pathogen-repo-ci.yaml, add an input to be passed to jobs.build.runs-on.

Example in d4fa54a is built on v0, but similar changes can be made to the latest version of the pathogen-repo-ci.

[joverlee521]

Seems like we can do something similar to the runtimes input

1. Allow specifying multiple in YAML format in input

.github/.github/workflows/pathogen-repo-ci.yaml

Lines 46 to 56 in 429f042

	runtimes:
	description: >-
	List of Nextstrain runtimes under which to run the build, as a string containing YAML. This is easily produced, for example, by pretending you're writing normal nested YAML within a literal multi-line block scalar (introduced by "|"):
	with:
	runtimes: |
	- docker
	- conda
	Defaults to "docker" and "conda". One job per runtime will be run.

2. Parse YAML into JSON

.github/.github/workflows/pathogen-repo-ci.yaml

Lines 150 to 158 in 429f042

	- id: inputs
	env:
	runtimes: ${{ inputs.runtimes }}
	shell: bash
	run: |
	runtimes="$(yq --output-format=json --indent=0 . <<<"$runtimes")"
	echo runtimes="$runtimes" | tee -a "$GITHUB_OUTPUT"
	outputs:
	runtimes: ${{ steps.inputs.outputs.runtimes }}

3. Parse JSON into matrix for the build job

.github/.github/workflows/pathogen-repo-ci.yaml

Lines 167 to 172 in 429f042

	nextstrain-build:
	needs: [configuration, workflow-context]
	strategy:
	fail-fast: false
	matrix:
	runtime: ${{ fromJSON(needs.configuration.outputs.runtimes) }}

[victorlin]

Seems like we can do something similar to the runtimes input

+1 for this compared to my prototype which defines the list of runners on the matrix with the repo name. It makes more sense to have runners x runtimes, although the similar terminology may be confusing!