Better taskcluster-gha branching

[dylanhand]

Improve using branches with taskcluster-gha.

Currently it's possible to use a branch, but it's a bit of a hack and only works if the branch is specified in the very first call to a given taskcluster-gha repo.

This PR makes it possible to specify a branch on a per-gha call basis. This means multiple branches of the same taskcluster-gha repo may be used at the same time.

Usage looks like this:

...
# Since no branch is specified in the `GithubAction` below, `main` will be used
.with_gha(
    "bundler",
    GithubAction(
        "divvun/taskcluster-gha/speller/bundle",
        {
            "speller-type": type_,
            "speller-manifest-path": "manifest.toml",
            "speller-paths": "${{ steps.build_spellers.outputs['speller-paths'] }}",
            "version": "${{ steps.version.outputs.version }}",
        },
    ).with_outputs_from(lang_task_id),
)
# This action will use the `windows-codesign` branch
.with_gha(
    "codesign",
    GithubAction(
        "divvun/taskcluster-gha/codesign",
        { "path": "${{ steps.bundler.outputs['payload-path'] }}" },
        # TODO: remove branch when done developing
        branch="windows-codesign",
    ),
)
# This action will use the `other-branch` branch
.with_gha(
    "deploy",
    GithubAction(
        "divvun/taskcluster-gha/speller/deploy",
        {
            "speller-type": type_,
            "speller-manifest-path": "manifest.toml",
            "payload-path": "${{ steps.codesign.outputs['signed-path'] }}",
            "version": "${{ steps.version.outputs.version }}",
            "channel": "${{ steps.version.outputs.channel }}",
            "repo": "https://pahkat.uit.no/main/",
            "nightly-channel": NIGHTLY_CHANNEL,
        },
        # TODO: remove branch when done developing
        branch="windows-codesign",
    ),
)
...