GHA: make element-web and cypress workflows reusable

[richvdh]

Part of element-hq/element-web#25313.

The idea here is to allow us to fire off a Cypress test run against a rust-crypto-enabled Element Web R after each js-sdk PR. These need to run in the context of a js-sdk PR, so we could just copy-paste the workflows into the js-sdk; however a more pleasant alternative is to make the existing workflows re-usable.

---

This change is marked as an internal change (Task), so will not be included in the changelog.

[t3chguy]

Surely this could be driven by just choosing that repository to workflow_call an action upon?

[richvdh]

Sorry, I don't understand?

[richvdh]

To be clear: I don't believe there is any way to figure out which repo this workflow file has been sourced from, from within the workflow.

[t3chguy]

Sure, and each repo/fork could just hardcode their own repo - https://github.com/matrix-org/matrix-react-sdk/blob/develop/.github/workflows/tests.yml#LL36C21-L36C21

[richvdh]

we've discussed this at some length in the chat, but to summarise my position:

One way or the other, we have to hardcode matrix-org/matrix-react-sdk twice. We can either:

• Put it in the calling workflow twice (as matrix-js-sdk#3412 does), or:
• Put it once in the calling workflow, and once in the target workflow.

There are a couple of approaches to the latter, but whichever approach we take, my contention is that, given the need for duplication, it's better that the duplicates be next to each other than at a distance. Primarily, doing so makes it much clearer what needs to be changed when setting up a fork.

---

Looking at this from the PoV of the calling workflow, we either have:

1. @t3chguy's proposal:

   jobs:
     build-element-web:
       uses: matrix-org/matrix-react-sdk/.github/workflows/element-web.yaml@HEAD
       with:
         matrix-js-sdk-sha: ${{ github.sha }}

   It is unclear to me, looking at that incantation, that even if I change the reference in uses, it will still build the react-sdk from matrix-org/matrix-react-sdk.

2. @richvdh's proposal:

   jobs:
     build-element-web:
       uses: matrix-org/matrix-react-sdk/.github/workflows/element-web.yaml@HEAD
       with:
         matrix-js-sdk-sha: ${{ github.sha }}
         react-sdk-repository: matrix-org/matrix-react-sdk

   Yes there is duplication there, but it saves hardcoding the repository in the target workflow, and it is pretty obvious what I need to change if I want to use a fork of the react-sdk.

[t3chguy]

a third approach is to split out into a composite action which the js-sdk and other callers can call, and it'll have access to its own repository via github.action_repository where no duplication or hard-coding is needed

[t3chguy]

Same as above

[andybalaam]

The repetition Rich proposes looks almost 100% harmless, and the code looks good otherwise.